For the amount of water you're talking about using up with that size tank....just get an RO/DI unit. It will pay for itself quickly! Also, a couple of BRUTE trash cans from homedepot with lids, set up with floats, and you always have RO/DI water ready to go. Then, you can use one for top-off water, and mix salt water in the other for water changes. It also gives you readily available stores for "emergency" water changes. And again....it will QUICKLY pay for itself vs buying ro water.