An airstone is only going to aid you if you have poor circulation and for stripping of CO2 from the water. If your running CO2, it wouldn't hurt to run an airstone at night to prevent CO2 from building up. No matter what the tech level, heavy fish load is going to produce algae. Shrimp on the otherhand maybe not. Yeast reactor CO2 can work. You can't use like a ceramic disc diffuser as it's not high enough pressure, but you would have to find an effective way of injecting it into the water (there are some commercially available products out there). ON a tank that size, you would need probably 4-6 2 liter bottles, you can have them all plumbed into one line. The more bottles the better, as each bottle will probably last you around 3 weeks before it should be restarted. IF you stagger the bottle resets, you can get pretty stable Co2 production from this method. Or if you can't get that going, dosing excel could work too-or get plants that don't demand CO2, set a conservative photoperiod, and keep the fish stocking really light.