I started out not using RO-di.. I didn't know any better at the time.. I always did fresh and always used treated tap.. I learned pretty quickly that RO-DI was needed when I started getting all sorts of algae growing off everything.. I've been using store bought since then.(un) luckily when my tank broke, I started with new sand and water and cleaned the rock that I put in and haven't had an issue since.. I did lose a few fish when using tap.. haven't had nearly as many losses since then (just one or two due to jumps) and my mandarin that stuck his head into a hole too small for him..