Turn off your filters then directly squirt excel on to any and all BBA you can find. Wait for a minute or two then turn your filters back on.
The BBA will turn out red/purple in a day or two and will die off. If the BBA returns, it means you still don't have enough CO2 for the amount of light you're using.
As for otos, I'm pretty sure they can handle high amounts of CO2 as long as the plants are photosynthesizing well and producing high amounts of O2. In my tank, the pH drops all the way down to 5.7 when CO2 is injected (and controlled by pH controller). If I put aside tank water and let it de-gas for 48+ hours, the pH is at 7.0. If you go by the CO2=3*10^(pH drop) formula, it calculates to 59.8ppm. That's basically double the 30ppm normally preached.