100% distilled water would not be good for fish long term because it is lacking trace minerals that they need to survive. It also gets quite expensive unless you have your own still or reverse osmosis unit.
Some people use distilled water mixed with tap water (usually 50/50) if their tap water is unusually high in nitrates or other environmental contaminants. Others mix defined salt formulations into distilled water in order to replicate specific water parameters like Lake Tanganika or Lake Malawi. These salt mixes contain all the trace goodies that distilled water lacks.
Unless you have particularly crappy well water or are breeding Rift Lake cichlids, you should be able to get away with tap water.
The only additive that you really need to add is a dechlorinator. Look for one that removes chlorine, chloramine, and heavy metals. All of the products on the market that remove these three things are fine. Some have extra bells and whistles like slime coat enhancers. They're all pretty good, IME.
Whatever you do, don't ever use chemicals to change your pH. They work for a short time until they're exhausted, and then the pH will swing back to its original level. Most fish are pretty tolerant of stable suboptimal pH, but intolerant of rapid pH shifts.