There was a debate on this a while back. Some conditioners say in their instructions that you should add enough for the whole tank. The only reason we could see for this was that it would take longer for the dechlorination process to take place if you only add enough for the water you add, and perhaps in that small amount of time the fish would be exposed. I usually only add enough for the water I add, I don't believe in dumping extra chemicals into the water on a regular basis if I don't have to, and my fish seem fine. My water also isn't very heavily chlorinated either.