i have a 100 gpd, and although its slowed down a lot its still plenty for my 90. BRS sells an upgrade to their system that upgrades from 100 to 150. But, if you know how ro/di systems work, you end up putting another di resin unit on the waste water, to add some gpd to your overall flow. Thats great, except the waste water is seriously taxing on that other di unit, and creates back flow to the first di unit with a y connection. So, I'd stay away from the 50 gpd upgrade, and stick with the 100. Best part about the BRS system is that the filter changes are a lot easier, and you can pack them (di and carbon) yourself, so you save money in the long run. If you have more $ though, there is an ro/di unit i read about on reefkeeps, with an auto back flush system, cleaning timers, built in tds meter, and other goodies that all comes packaged in a little black box (think it runs around $1k, though not sure). That would be the pinnacle of ro/di units that i've seen. Except for maybe that 10k gpm thing on ebay.
