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I have several Blue Gouramis as well as Silver dollars, Giant Danios, and a Red-Tailed Black Shark in a 55 gal. tank. I try to use RO water during my water changes and then top the tank off with tap water. I would be interested in feedback as to which is better, RO water or ordinary tap water. Thanks, Pat

RO water is 99.9% pure water. It contains no minerals, buffers, vitamins, etc... For that reason it cannot be used by itself in a fish tank. The ph would crash really quick. Tapwater is variable. It contains a wide variety of things and some tapwater is no good for fish. Probably the best thing for fish would be to start with RO and then used powdered minerals to create the exact water parameters you want with no chance of contamination like you find in tapwater. However most people don't have the money, the time, or are willing to put in the effort to do that. I've tried and it's a pain unless you've got really small tanks. You can however mix tapwater and ro with every waterchange so that your tapwater is softened and diluted but the resulting water still has the minerals and buffers needed. Make sure you test this mixture for kh and ph and try to make at least 3-5dkh(degrees carbonate hardness) or your ph may crash.

As for topping with tapwater your kinda doing it backward. When water evaporates pretty much only h2o evaporates leaving behind everything else in the water. This makes the hardness of the tank increase. Therefore you want to use softer water as top off water. Most use tapwater in their tank and then top off with RO water. The problem is with you already using RO water in the tank topping off with tapwater is probably what's saving your fish. Your adding enough buffers back in to keep the water stable. I'll repeat: you can't use only pure water(ro/distilled) in a tank with nothing else added. The tank will crash.