Originally Posted by LittleBettaFish
Your water can never be too clean. Ammonia and nitrite are extremely toxic, and you only have to see how fish react to their presence to know this. Fish can't build an immunity to these, and the only reason fish succumb to disease in poor water conditions is because the stress of exposure to ammonia/nitrite weakens their immune system.
I've come to the conclusion that all these threads asking about water-change frequency are missing the point.
Until someone with impeccable credentials and experience corrects me, this is my answer to, "How often should I change the water in my (whatever size) tank?"
Whenever it tests at .25ppm ammonia. (Can I put that in caps, bold and underlined?)
How much to change is where the discussion should begin, IMHO.