Some common reef chemistry misconceptions


Reef Hacker
There are many posts made asking some basic reef chemistry questions and sometimes the questions themselves or the answers given are innacurate or based on faulty testing. I'm not a chemist, but I have been in this hobby long enough and have read enough to have a good understanding of the basics of reef chemistry.

To start with, I will list the average, major parameters of natural seawater on most reefs around the world. Under most circumstances, there is no reason to keep our water parameters anything different than what is natural. There is a nice range for each parameter and no one should have any problems keeping their water in those ranges.

calcium: natural - 420, recommended range - 380 to 450
alkalinity: natural - 7dKH, recommended range - 7dKH to 11dKH
magnesium: natural - 1280, recommended range - 1250 to 1350
salinity: natural 34ppt to 38ppt (1.025 to 1.029 SG), recommended range - 34 to 35ppt
pH: natural - 8.0 to 8.3, recommended range - 7.8 to 8.5
temp: natural - 80 to 86, recommended range - 78 to 83

Some of the most common questions/answers/comments I see.....

"You need to use a Reef Salt for corals"
Every major brand of salt, when mixed at the recommended salinity, has parameters within or sometimes above all the recommended ranges. What makes one brand a "Reef Salt"? Manufacturer advertising. Some salts have much higher than average calcium. That has no affect on making corals grow faster. Some have higher than average alkalinity. While studies have been done showing some corals grow faster at 8 dKH rather than the natural 7dKH, anything over around 11dKH does nothing other than push the saturation levels closer to super saturation and closer to a precipitation event. Some corals can also get "alk burn" where their tips start bleaching from high alk levels. Some salts claim to have "vitamins" added (such as Reef Crystals), but the fact is no one knows exactly what those vitamins are, if they are actually needed and how long it take for them to break down or what they break down into. Every major brand of salt contains all the needed "trace elements". Personally, I use either regular Instant Ocean or regular Red Sea salt. Both mix to parameters very close to natural seawater.

"Aragonite sand buffers your water"
Aragonite sand is calcium carbonate. Calcium carbonate is what hard corals form to make the skeletons. For aragonite sand to do any buffering, it must dissolve. For it to start to dissolve, your pH must be around 7.4 or lower. Under most circumstances, the pH in a reef tank will never get as low as 7.4. If it did, your corals would be dissolving along with the live rock and sand. The only place in a tank you might see a pH that low is deep (6" or more) down in a deep sand bed. The sand may start to dissolve, but the saturation level is so high in that localized area that the calcium and carbonates that resulted in the dissolution will precipitate back into calcium carbonate. The water column gains nothing. Just think of how a calcium reactor works. You pump co2 into the reactor to lower the pH so the calcium carbonate media can dissolve and that enriched water is pumped into the water column.

"If you raise your calcium, your alk will drop"
As long as your alk and mag levels are within the recommended parameters, adding a calcium supplement does one raises calcium. Under extreme circumstances, where your alk is already very high, your mag is very low (mag helps bind the calcium and alkalinity) and you push your calcium to a very high level, if your pH is also very high (8.5 or more) it can cause a precipitation event which will drop the alkalinity.

"If you raise your alkalinity, your calcium will drop"
Same as above except pushing alk too high can also temporarily raise your pH which can help bring on a precipatation event. The higher the pH (8.5+), the more chance there is for the super saturated calcium and alk to precipitate. They are more stable at a lower (8.0) pH.

"I have to dose alk, but my calcium never drops and I don't need to dose that"
When calcification occurs (corals forming the hard skeleton, coralline algae growing or abiotic precipitation) there is a defined amount of calcium, alk and mag used. That rate is aprox. 18ppm calcium, 2.7dKH alk and 2ppm mag. Some corals use more mag and to a smaller degree strontium, so that rate can change to 16ppm calcium, 2.7dKH alk and 4ppm mag. Many people like to use the cheaper, lower grade test kits (API is a perfect example) which are not very accurate at measuring calcium. If I'm not mistaken, they measure in increments of 20ppm for calcium. Are you sure when you added that last drop the color change was at 420ppm, or was it actually 400 or 440? Another factor is many people use a salt that is high in calcium. Every water change adds to the calcium level so you may not see that drop.

"I tested my calcium and it was 200ppm"
I won't say impossible, but it is naturally impossible. Using the fixed ratio shown above for calcification, if your water was 400 and dropped to 200 that means your alk dropped 27dKH. Since there isn't that much alkalinity in the water (recommended level 7-11 dKH), the calcium can't drop that low. How it can get that low is if you have a precipitation event and only add alk, precipitate again and only add alk, etc, etc. Most common reason for seeing that low of a calcium result...testing error.

"My pH is 7.4...or lower"
Testing error. Under normal circumstances, with your water parameters in the recommended range, the pH can't drop that low. The buffering affect of the alkilinity prevents it. The most common reasons for lower pH (7.7-7.8) is the air in your house has alot of co2 in it and that air equalizes in the tank or you have alot of dying organic matter releasing co2. But, if your pH was actually that low you would see your live rock, aragonite sand and corals starting to dissolve since all are made of calcium carbonate which starts to dissolve at that low pH.

"I added a buffer that is supposed to keep my pH at 8.4, but it keeps dropping"
Buffers that claim to help keep your ph stable are a mixture of sodium bicarbonate (regular baking soda) and/or sodium carbonate (baking soda that has been baked where the heat drives out the excess co2) and borate. It is natural that your pH will eventually fall back to whatever it was before adding the buffer. As long as your parameters are in the recommended range, your pH will be pushed down by either a high co2 level in the air in your house or a high level of dissolving organics in the tank. The borate also has the added affect of messing with your alk test since it can force the test kit to show a higher alk level than you actually have. The best fix to maintain your pH in the recommended range is to open a window (if possible) to get more fresh air in the house and/or run a high quality carbon to help remove the organics pushing down the pH.

I'm sure there are others I may have forgotten, but I hope this helps clear up some of the concerns, questions or misunderstandings in the chemistry part of keep a healthy reef tank. For some more detailed and scientific explanations, here's some great articles to read.
Reef Aquarium Water Parameters by Randy Holmes-Farley -
A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH by Randy Holmes-Farley -
Chemistry And The Aquarium: Solving Calcium And Alkalinity Problems — Advanced Aquarist | Aquarist Magazine and Blog
Calcium and Alkalinity by Randy Holmes-Farley -
Chemistry And The Aquarium: The Relationship Between Alkalinity And pH — Advanced Aquarist | Aquarist Magazine and Blog
Aquarium Chemistry: Magnesium In Reef Aquaria — Advanced Aquarist | Aquarist Magazine and Blog
Chemistry And The Aquarium: Boron In A Reef Tank — Advanced Aquarist | Aquarist Magazine and Blog
Really Helpful post, I was wondering about the salinity range that you have listed. I always thought that it was supposed to be much lower then that. I was actually about to make a post about how to lower mine because it was usually around the 1.028-1.030 range. So are my levels ok?
Thanks for the tips! So how do I calibrate it, it came without any kind of directions so I am kind of in the dark. I have goolged it somewhat but I would rather get my information here.
just put the solution or ro water on the glass and make sure its reading 0 for ro or 35ppt for a reference solution
adust as needed