Hello! I've been running several BRS and Spectrapure RODI systems for a while, and I too deal with chloramines. My water co. adds up to the legal limit, <4 ppm if I recall correctly, water report states 3.5+ ppm. I have added on an extra canister to a couple of my units to try to maximize my efficiency in removing chloramine.
My setup is as follows for my tank water units -
1 micron sediment filter -> 1 micron BRS Universal Carbon Block -> 1 micron BRS Universal Carbon Block -> RO -> DI.
I still add just a dash of Prime before putting remineralized water in my tank. Better safe than sorry.
For drinking water purposes, setup a bypass for DI. It's not technically recommended to drink RODI water, but RO is fine.
This has provided me with excellent filter life, including the DI, and undetectable levels of chlorine for many months. Test strips test total chlorine, which is what chloramine would be detected in, and free chlorine. I do not bother using standard granulated activated carbon blocks in my units because chloramine exhausts them VERY rapidly in my experience. You have to have catalytic carbon.
Two things will occur if you do not have catalytic carbon (Universal Carbon Block) to break the chlorine+ammonia bond. 1. You're adding unnecessary wear and tear on your RO membrane, as there will be chloramine breakthrough, and 2. You'll rapidly degrade your DI stage as it tries to deal with the bypass chloramine. The RO membrane is not at all effective at chloramine removal.
As to allowing Ca and Mg through, there's no way to do that. You'll have to get a GH booster. I use MgSO4.7H2O and CaSO4.2H2O for remineralizing, plus a dash of CaCl. I'd be happy to share my chart with you, might make life a little easier.
For drinking setup, I have two of these, one at my office and one at home and they're great -
This would be my recommendation, and then have a separate storage for your tank water. This already has an auto shutoff and a flow switch, so you would just need a float valve and a storage tank for fish water. 55 gallon blue storage tanks can typically be found pretty cheap on Craigslist! The pressure tank is designed to fit under your sink, help keep out contaminants, and give you a nice flow rate to fill your glass. If I were trying to fill buckets, however, at the flow rate my faucets have...I'd lose my mind. That 30 gallon says it has an 18 gallon water capacity, seems small, and you won't be able to adjust the flow rate of water - your stuck with however fast it wants to give you water.
If you have any questions, feel free to PM me.