Hello; I keep zebra dianoes in quarintine tanks to keep down the mosquito larvae. I sort of gathered that the OP was using an empty tank to store extra water and that they may not have wanted the water to be always ready to hold fish. Perhaps I miss read. I store water for changes in containers for at least a week to allow the chlorine to dissipate in order to avoid the use of chemical conditioners. Perhaps the OP dumps straight tap water into the storage tank from time to time?
Note - My tap water does not have the other additives that do not dissipate and am saying this to warn that all tap water cannot be aged this way.