I want to get soft water fish and I live on a bed of limestone so my tap is liquid rock. I have a no income budget so have to save for very long time for RO unit. Could I collect rainwater for the tank and then in the future switch over to RO? Or would this shock the fish? Also, could I let the rainwater sit for up to a few months before I need to use it in a 55 gallon barrel or would it go bad/stale?