Just saying---there are a few contaminants that are lethal to fish at levels rated safe for us to drink, and evaporation can build them up in a tank: there's one we ran into in OKC that could be a problem. You might talk to your local university's head of chemistry and ask. As a former teacher, I received questions from the community now and again and was pleased to help, in my own field, which is not chemistry. I would consider a safe fill followed by safe topoffs and maybe a few really good supplements to hold the line until the lads in white coats can give you better information---problem is, if your grocery store's bottled water is filtered from local taps, that's not going to help you. Well water stands a chance of being contaminated if ground water is.