StressZyme is nothing more than 'bacteria in a bottle' used mainly to start the biological filtration process in your aquarium. Some use it during water changes to help their aquariums if they lack the proper live sand bed and live rock but will do nothing for contaminants in the water. StressCoat is used to remove chlorine from the tap water but will not remove any other contaminants that is common in tap water. RO/DI vs. tap water??? To put it in terms for the average person to understand who isn't an aquarium buff, an RO/DI unit will remove roughly 99% of everything that you do not want to introduce into your aquarium to prevent a very wide variety of issues. Most tap water systems will have a wide variety of contaminants including heavy metals and phosphates among many other things which an RO/DI unit will remove before adding the water to your tank. I am a rare exception to the tap water rule. I live in a town where the tap water is so unbelievably clean that I haven't used an RO/DI unit in 20 years with success that will make most jealous. But I am definitely an exception to the rule, the water system in the next town 10 miles away has heavy phosphates in their tap water which create a few problems to say the least. My friends tank in another city 30 miles away would completely crash within a couple of weeks if he didn't use an RO/DI. Each towns water system is different. If you are in doubt or are having problems, get a good quality RO/DI unit. I would always recommend one to anyone just starting out or someone who is having water quality problems. You definitely will not hurt anything by using an RO/DI unit, only help prevent problems. Water changes... unless you have contaminants in your tank you are trying to get rid of, they are to remove the Disolved Organic Compounds that build up in your tank over time. If you are having constant nitrate problems, you might want to make sure you have a proper sandbed established for you biological filtration with proper water flow/oxygen exchange.