I'm a little confused, unless I'm missing the obvious here. Is your water turning into POISON? If so, what exactly is "poison" to you. Are they dying off one by one? Learn the NITROGEN CYCLE if they are.
I don't know what your definition of perfect is, but mine is never doing water changes and having everything together happily. But that never happens, so I do water changes and keep fish away anyway, lol.
And the "running back to the store" - this is why a lot of people usually buy basic medication at the very start. I don't have a large list (I hate use of medications, but I still have some in that kind of emergency!) but there some good basic ones. Lol. Is this your first fish tank?
And how about a full, entire list of your fish, with head-counts on how many there are.