My RO/DI filter was a water general from ebay. I think it ended up running me like $140 for a 6 stage filter...all in all, not a bad price. Make sure you get a pressure gauge on it, as they come in handy for sure. Inline TDS meters are also helpful. A few things to consider...
1. Depending on your water authority, anywhere from 30-70% of your TDS is calcium. We pull it out of our water, just to put it back in. That said, chlorine and chlorimides (which stay in water for 3-6 weeks before burning out) can cause big issues in large enough doses. Elevated Nitrate and Phosphate levels, as well as copper from pipes in the house, and other dangerous to reef life metals can be in your water.
2. I don't risk it, but have certainly seen a lot of people who do without negative effects.
3. I get a film on the top of my water from dosing 2 part if I'm lazy and overdose the calcium without keeping up on my magnesium. I precipitate a small amount of stuff into the water and it accumulates on the top of my tank for some reason. Do you test for magnesium, or use two part?