If you have little money to spend then spend it on testing first. The beginning of preventive maintenance is knowing what your working with. You can not begin to know what you can or can not afford to do for your tank and its inhabitants unless you now what needs to be done. Not doing regular testing can be the most expensive mistake you can make, and it takes little time or money. Ph and salinity should be tested as frequently as is practical, but at least once a week, daily is better for your pH. Depending on your fish load testing for nitrates should be done at least weekly, and nmmore often if you mix fish with invertebrates and stoney corals. Anytime you add any livestock, sand or rock means you should test for ammonia daily for at least three or four days. If you have a high stoney coral load a weekly check of alkalinity and calcium and phosphate is a minimum (a good time to do that test is right before your weelly water change so you can calculate how much calcium your using or needing to add and can adjust your new water for any needed alkalinity boost your tank may need). And if you have a high stoney coral load or really thick corraline growing, the weekly testing is only after you have tested frequently and often enough to know your calcium and calcium carbonate demands. Magnesium is tested for less frequently usually once a month unless you use extreme amounts of calcium and carbonates. Most people do not test for iron, boron, silicates, iodine, or strontium. Seldom is copper tested for unless copper medication is used. Remember any changes from normal, such as adding stock, changing feeding amounts are types of food, changes in brands of salt, using different supplements, switching water sources, even changing lighting can cause a need for more testing than is your routine.