How to get perfect aquarium water?

So I have a 10 gal with two bettas (and a divider!) my one betta stopped eating, started hanging out at the bottom, etc., so i checked ammonia level, nitrate/nitrite, pH, water hardness, etc, and everything was as it should be. when i moved him to a bowl, i noticed that his scales are raised, so he has dropsy :( my question is, how can i prevent it in the future? all the levels were correct...and i'm about to remove the other fish into a bowl so she doesn't get sick, but i'm nervous about the tank in the future.