Both the brass or any metal fittings and the pressure tank are bad ideas.

Brass contains copper which can be deadly to reef inhabitants. On the tap water side its not a problem as most tap waters have a higher pH, low dissolved oxygen and what is called a positive Langlier Index. This means the wate rnot agressive and tends to scale or build up on pipes whcih is the opposite of RO and RO/DI water which is very agressive and can eat pipes and metals trying to gain back some of the ions that have been filtered out. Stainless is not safe either as the RO/DI will eat it too, just not as fast as the copper or brass and with less traumatic results. The only metals safe with RO/DI are titanium and tantalium that am aware of.

The pressure tank is not a good idea as they make a RO system cycle onand off more frequently to replenish the lost pressure when you take water from the bladder or pressure tank. This leads to TDS creep which eats DI resin much quicker than long filter runs which cleanse the membrane lowering the TDS. Also RO/DI will leach things out of the rubber bladder causing even more problems.

If you are set on a pressure tank, which I would discourage from personal experience, store only RO water in it, drain it 100% often so it refills completely with fresh RO and reduces the TDS creep effects and place the DI on the outlet of the pressure tank but with a valve to regulate the flow through the resin so you get proper contact time with the DI resins for best treatment. Too fast through the resin and it wil not be 0 TDS.