90 gallons probably won't see any good or benefits from DIY co2. I'm currently setting up a 90 gallon tank as well. Have been looking at lighting. I'm shooting for medium high light. But to get it I'm going to order a custom-built light from Catalina. 4 bulb T5HO with roughly 8 inch separation between the pairs. (thanks for the recommendation hoppy)
The issues with bigger tanks like the 90 gallon is they are deep with decent width. So to get coverage of light throughout the tank to the bottom.
If your looking for cheap but still get low to medium light, you could look at an Odyssea fixture 4 bulb T5HO set up for just shy of $100.00. It will be lower light.
Sun Sun pimp #72
RAOK CLUB # 68