I'm not even sure if red root amaranth is an aquarium plant. Nevertheless, here are some general tips:
1. get pH, gH and kH liquid test kits
2. You can estimate the co2 concentration based on the pH and kH values (google ph kh co2 for a chart)
3. Increasing the rate of co2 diffusion from 5bps to 6 bps is a significant change, so be very careful, especially since you mentioned that you think its already affecting your fish
4. I wouldn't worry about Ca concentration unless your gH is less than 5.