Spectrapure 4 stage for me, like a lot of my stuff got it second hand from a reefer that upgraded.
Inline TDS meter, pressure gauge, auto shutoff valve, and flush valve are all useful. Membranes are the part you need to take note of, rejection rates will determine the tds of output water. 98% vs 99% and then GPD of the membrane and waste water. If you have low water pressure then a booster pump is required to get anywhere near the published GPD. I think BRS found 90 PSI is optimal for the correct performance. I have 78 PSI and don't use a booster pump.
For planted tanks were lucky and don't need 0 TDS, I don't run DI for that reason, with such large water changes the DI depletes quickly. If you do use DI replace it regularly, it can leach the weakest bonded ions back into the good water when stronger bonded ions replace them.
Know if your city uses chlorine or chloramine and use the correct carbon block setup to save your membrane.
Use auto shut offs, if you don't you will have wet floors at some point. BRS videos will teach you everything you will ever need to know about RO units and how to make them the most efficient.
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