Water Chemistry

The best reefers in the world will tell you that they don’t keep fish or corals -they keep water. Water is the lifeblood of your tank, it is what all your critters live in, breath in and eat from. Some animals, like corals, pull nutrients directly from the water, other wait for it to go floating by. So if you want to have any sort of quality reef tank you must have exceptional water quality.


Many people have freshwater tanks, or once did and think that many of the rules apply to marine tanks. Most don’t, saltwater fish are much more sensitive to water quality, and corals and other invertebrates are even more sensitive. What works for freshwater probably doest work for a marine one.

Making Up Saltwater

So, lets start with the very basics. What is good water to make up my saltwater with?
Your tank water should be made from reverse osmosis and de-ionized water (RODI water) or distilled water. You can buy these from your local grocery store, LFS, water depot or you can make your own. The filter that makes RODI water should has at least 4 stages and costs between $100 - $200 depending on how many stages it has. This filter will remove all the additives and dissolved minerals in your tap water, producing nearly 100% pure water. The filter works by reverse osmosis, so there is some rejection water that is not fit for consumption (but can be recycled for watering plants and such) and can only produce so many gallons per day. The filter should be replaced regularly, and a TDS meter should monitor the quality of water coming out. You can find these meters for very cheap on ebay. They tell you the total dissolved solids in your water. Your RODI water should stay below a TDS reading of 10.

Places to find good RODI systems are:

FilterDirect.com Whole House Water Filters, Water Booster Pump Choices

You might be asking yourself why cant I use my tap water? Tap water contains chlorine and lots of dissolved minerals. If you test your tap water with your TDS meter you will often find readings in the 400-500 range, which is far beyond acceptable range for making saltwater with. One of the most common dissolved minerals in tap water is copper, especially if you have copper pipes. Copper is fatal to invertebrates (your bacteria, corals, shrimp, crabs, anemones etc) in minuet quantities. You would also need to add a decholorinator to your water, lessening your water quality and it can interfere with the proper function of your protein skimmer. Another common dissolved substance in water is nitrates. These are plant fertilizers and will lead to ugly algae problems.

Starting with good water is a long step in the right direction to keeping good water.

What salt should I use?

There are many different brands of salt out there. Hopefully this goes without saying, but you cannot use table salt! There isn’t a best brand, just look for one that mimics the natural balance of the trace minerals found naturally in the ocean. This isn’t as important if you are planning a FOWLR tank. Many brands of salt have a particular reef blend that may be better suited for a reef tank.

Putting it all together

When you mix up your saltwater you are going to need 4 things: a bucket, a powerhead, a heater and something to measure the salinity with. The bucket can be as big as you like, many people make up enough water for several water changes and then store it until they need it. You should change out 10-15% of your tanks volume every week with new saltwater. When you make up your water add your RODI water to your bucket and then add your salt. Then add your powerhead to the bucket. The powerhead will move water around, making the salt dissolves and spreading it evenly throughout the volume of water. The water movement will also oxygenate the water, making the saltwater less caustic. Newly made saltwater is very caustic and can kill your livestock. Always let the saltwater sit with the powerhead in it for at least 24 hours.

The last thing to check is the salinity of the water you made up. If you are doing a reef tank the salinity should be close to 1.025 (35 ppm). If you are doing a FOWLR tank you can have your salinity a bit lower, around 1.021. This will lower the amount of salt you need to use and will help keep some of the harmful parasites, bacteria and other pests from thriving in your tank, but you do need to be careful to keep the level from not dipping below where saltwater fish can comfortably survive. When you do a water change the water that you add to the tank needs to be close to the salinity of the water already in the tank. If the salinity is too different and there is a huge change in overall salinity of the tank it can kill the inhabitants of the tank, especially the more delicate creatures such as corals, shrimp, crabs, starfish etc . . .

There are two different ways to measure the salinity of your water. The first and best way is to use a refractometer. These are not commonly sold at LFS (or if they are they are usually really overpriced) but can be found cheaply on Ebay. Be sure to get the calibration oil that calibrates for saltwater to be sure your meter is reading correctly. This meter will last for years with proper care.

The other method for measuring salinity is to use a hydrometer. This is the meter most commonly sold at LFS but is fraught with problems. Even though they are cheap, they need to be replaces every 6 months to a year, and after two years it would have been cheaper to buy the refractometer. The hydrometers also rarely give accurate readings, and if there is any issue with the swing arm (not free moving, bubbles or crystals) it will give incorrect readings. Bubble will give higher than actual readings, crystals; lower. Also they are to be calibrated by soaking them overnight in saltwater, and if this is the first time you made up water, there is no way of knowing if your water is at the correct salinity which means your hydrometer is doomed to give false readings from the beginning. Below is a picture from a fellow reefer of two hydrometers, one 6 months old, the other brand new, each is filled with the same water.


Now that you have your water made up, you need to bring it to close to the same temperature as your tank water. Just like salinity, the temperature of water needs to be close to the tank temperature. Also salinity does depend on the temperature of the water so always measure the salinity of the water when the temperatures are comparable.

Congratulations! Your water is now made up correctly and you can add it to your tank!

It should be noted that many LFS sell premade saltwater which is perfectly fine to use, but you should still have a refrectometer to make sure that the salinity is close to what your tank’s salinity is.

Now my tank is filled with water, what now?

Once your tank is filled with water the more important part of water keeping come into play.


Keeping the temperature of your tank steady is the most important thing; actually temperature is second. Most reefers keep their tank around 79 degrees F. Some creatures, such as seahorses need to be kept at colder temperatures, like 70 degrees. Both higher and lower is ok, just so long as the temperature doesn’t change more than 2 or 3 degrees per day. Any swing larger than that can be deadly, and is probably one of the more common reasons for tank crashes, especially during the summer. Also heaters are very prone to breaking, and probably break more than any other piece of equipment. Because of this many reefer have two heaters in their tank, usually each is rated for half the volume of their tank, so together they heat the tank correctly. Buying a quality heater also helps. You should look for a heater that you can set the temperature on and has no exposed metal parts that can corrode in saltwater.

Water movement

In a reef tank lots of water movement is critical. In the ocean the water is constantly moving with force and corals need this movement to flourish. Water movement also distributes nutrients and removes waste from the area around your corals. Most importantly the water movement keeps oxygen diffusing into the water, and carbon dioxide diffusing out. If the water is stagnant your fish will quickly use up the oxygen in the water and they will suffocate. Some corals, like Small Polyp Stony (SPS) corals require extremely high flow, other like Large Polyp Stony (LPS) require less flow but still need moderate flow.

Water movement in your tank will be achieved by using powerheads. It is much better to have many small powerheads than a few large ones. You should have 25 -50 times the volume of your tank turned over by the powerheads per hour. So long as your sand isn’t flying around the tank, you don’t have too much flow. At least one your powerheads should be aimed at the surface of your tank creating a strong ripple, this will make sure your water is being oxygenated. (It should be noted here that if you have some sort of film on the surface of your tank it can interfere with oxygenation and can hurt your tank. It can be easily removed by placing a paper towel on it allowing it to be soaked up, or aiming more powerheads towards the surface drawing that “stuff” into the water column.) Any place that doesn’t have good water movement will become a nutrient trap, where stuff can settle out of the water column. These areas will usually have algae problems, so good flow over the whole tank will help prevent algae issues.


Ammonia, Nitrites, Nitrates and Testing Your Water

Every fish tank must cycle before you can add ANYTHING! Cycling the tank means to grow the bacteria that will process the fish poop into something much less toxic. The actual poop isn’t toxic, but it breaks down into ammonia (NH3) that is deadly to everything you are going to want in your tank. Thankfully there are bacteria that break down ammonia to nitrite (NO2-), unfortunately this is just as deadly. Together these are known as the fish killers, although they also kill everything else. Thankfully another type of bacteria will come to our rescue again by converting nitrite to nitrate (NO3). This is much less deadly, but at moderate levels it will kill most corals and at high levels will kill your fish also. The best way to start the cycle on your tank is too add some table shrimp or fish food and let it rot in there for a few days. Don’t do water changes until the NH3 and NO2- are gone, that will only lengthen your cycle. If you used a large amount of live rock and even with the shrimp in there you don’t see a spike of NH3 or NO2- , it is ok. It just means you skipped your cycle and have plenty of bacteria in there to handle fish waste.

So how do I know if I have these compounds in my water? By buying tests of course! There are a few methods for testing for them, but the best are the liquid tests. There are dip strip but they are highly unreliable, and should not be trusted. A good brand is the API test kits and Salfert, but they are a little more expensive.

Ammonia in an established tank should always be zero. If you have any ammonia you should immediately try to locate the source (look for something that died) and immediately do a water change (good reason why to always have water on hand and ready to use). However if you are cycling your tank you should see a spike of ammonia, and then it should go to zero.

Nitrite is like ammonia, you shouldn’t see it in your established tank (danger if you do!) but should see a spike of it when you are cycling your tank.

Nitrates are the end result of the nitrogen cycle (ammonia to nitrite to nitrate). You should aim to keep them at zero, and there are many ways you can do this. Many corals and invertebrates will require no nitrates for them to live. The simplest way to keep your nitrates at zero is to prevent them from occurring by not overstocking your tank, and not over feeding your tank. What food your feed is also important, flaked and dried food contain nitrates, so try to stick with frozen foods. The frozen foods are also much better for your fish anyways. Other ways to reduce nitrates is use a protein skimmer and refugium combination or an algae scrubber. These are discussed below. Keeping your nitrates at zero will also reduce troublesome algae growth.


Calcium, Magnesium and other Trace Elements

Many demanding corals and some invertebrates like clams require other trace minerals, like calcium, magnesium. You should not have to dose these trace elements, doing regular water changes with a quality salt should keep these in proportion to allow your corals to thrive. You should only dose what you test for! It is frighteningly easy for you to overdose something and cause your tank to crash. An example of a tank that would need to be dosed would be a tank dominated by high growth SPS corals. This type of tank would probably need calcium dosing because the corals require calcium to build their skeletons as they grow.



Alkalinity is a property of the water itself, and it is directly tied to the dissolved ions in the water and pH. It is a very important part of maintaining good water quality and for coral growth. If you are having alkalinity issues or wish to know more about alkalinity, I suggest doing more specific research.


In a marine tank the pH should be around 8 -8.4, which is slightly basic. pH is slightly dependant on the light cycle, so when testing the pH of your tank it is better to test later in the day. A slightly fluctuation in pH is ok, but large swings (like all large swings) can kill your livestock. Many reefers run the lights of their refugium opposite of their display to keep the pH more stable. If your tank has a consistently low pH, you might look at increasing the gas exchange by increasing the flow in the tank, especially at the top.


This trace element is deadly to sensitive corals and can be the root of many problems. However this element isn't easy to test for because any algae growing your tank will take it up with in seconds of it becoming available. On average it only lasts 7 seconds in the water column, making the chance of being able to test it (they do sell tests for it) zero to none. If you do suspect you have a phosphate problem, indicated by sensitive corals dying or failing to flourish or algae issues, you need to get a GFO reactor to pull it out of the water column.

That is the very basics on what you need to test in your water and water care. Most reefers will move beyond the very basics by adding equipment, which will be discussed below.
Water Equipment:

Just to recap what equipment you must have and have already been discussed.
• Buckets
• Powerheads
• Refractometer
• Heater
• Basic test kit (NH3, NO2-, NO3, pH & alkalinity)

Most reefers will go beyond this for keeping excellent water quality, and here is some of the commonly used equipment.

Protein Skimmer

This is probably the most commonly added piece of equipment to any marine system, and one of the most beneficial. It works by removing solids from the water column before they can rot and reduce your water quality. When looking at buying a protein skimmer a good rule of thumb is “you get what you pay for”. Cheap skimmers are cheap skimmer and can be prone to flooding. A great brand for the money is the Reef Octopus skimmers. There are both hang on the back and in sump models.

Algae Scrubber

These are becoming very popular as alternatives to protein skimmers and have the added benefit of being very cheap and easy to build. They work by giving the hair algae a place to grow, and then it is cleaned once a week. This removes nutrients from the tank by removing the algae that grew from it. The wastes are allowed to grow through the nitrogen cycle, and the nitrates provide fuel for the algae to grow. The algae covered screen also provides a place for pods to live which are then pumped back into your tank to feed your fish. The algae also releases other nutrients back into the tank that the corals can use. An excellent thread on how to build one can be found below.



This is a dedicated place for algae to grow (removing nitrates and other excess nutrients for the water column) and also a safe place for pods to breed and populate before being pumped back into the main tank. It can also function as a handy place to banish predatory or unwanted creatures from the main tank. The most commonly used algae species used in the refugium is chaetomorpha. Refugiums are usually part of a sump, although there are some hang on the back modles. Remember that the algae does need a source of light, but a CFL bulb (the energy efficient squiggly ones) work just great.


A sump is a separate tank that holds a protein skimmer, refugium (most commonly both of them) and then a section to pump the water back to the main tank. A diagram of the typical sump set up is below.

2- Protein Skimmer
3- Refugium
4- Bubble trap
5 - return pump

UV Light

This piece of equipment is designed to kill parasites in the water column and to kill algae spores. This is not a common or necessary piece of equipment, but can be very useful for tanks that have fish that are disease prone, such as tanks. Set up for the UV light is very important, because the light can only penetrate a small amount of water, and if the water is fast flowing it might not work properly. If you do invest in this piece of equipment be sure to set it up correctly to get the full benefit. A word of caution, this should not be used as a crutch in a tank that is over stocked or has incompatible livestock and is suffering frequent disease outbreaks. Those problems need to be addressed before adding a UV light.


This is also used to kill parasites and algae spores in the water column, however this is a very uncommon piece of equipment. If used incorrectly it can easily kill your livestock, so I wouldn’t suggest using one.

Commonly used equipments that can cause you problems:


Filters are a freshwater concept that doesn’t translate well into marine tanks. The filters trap debris that rots down to nitrates and then becomes a major source of nitrates in your tank. They can be used on a marine tank, but they require constant maintenance. Don’t be fooled by your LFS, they love to sell you these because they are expensive and you have to continually buy the filter material.

Bio Balls

These are commonly seen in the marine world, and while they have a great idea behind them, they usually become more trouble than they are worth. The idea is that they would provide additional area for the beneficial bacteria to grow, however they end up becoming a nutrient trap and nitrate factory. Personally, I would slowly remove the bio balls (a few every water change) and put rubble in that area instead. Or you can keep a tight maintenance schedule, cleaning a few every week, and continue to use them.


Once you understand the basics of keeping water and have the equipment you decide to use I think you will find keeping excellent water is easy and will allow you have an amazing reef tank.

My personal opinion on keeping good water:

Fancy equipment may help a lot, but it doesn’t replace the need for good water changes and taking the time to do things correctly. This includes feeding your tank and how you choose to stock it. I would stay away from dosing unless you have a proven need, and even then be sure to research how to do the dosing correctly because overdoses are common and deadly! I would also stay as far away as possible from things that are said to reduce nitrates or kill algae. I feel its like putting a band aid on a deep cut. It might cover it up, but its not going to fix the problem completely.

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