Good to have you with us!
I would recommend the best RO or RO/DI system you can afford. Good water quality is, IMHO, the single most important thing you can do for your tank. In order of ascending benefit, the systems available are:
> Tap Water Purifier (DI only) > 2 Stage RO (Bare Bones) > 3 Stage RO > 4 Stage RO > any of the RO units will be enhanced by addition of the DI cartridge.
You may not need 100 gpd, but bear in mind, most of us aren't going to run the unit all the time so that the higher output, the less time needed to run the water you need.
Check our sponsors for specials and any other vendors for best prices. You can get a pretty good setup sometimes for less than $150.
The Tap Water Purifiers (I've been using one; RO/DI is on the way
) gives fair water at a lower initial cost. However, the cartridges run about $16 apiece and may be good for between 50-100 gallons before replacing, depending on your local water system quality. That adds up rather quickly to the price of a decent RO/DI!!!!