A couple of things stick out, but can you provide the information I’ve outlined, below?
In the meantime, based upon what you have provided, here are the early suggestions:
1) CO2 appears to be low. If your dKH and pH readings are correct, they indicate a reading of about 6ppm CO2 when fully gassed. Generally, we prefer to see about 30ppm in a high-tech tank. Using bps is not a good way to determine CO2 levels. To be more certain of achieving 30ppm, increase CO2 until you achieve a full 1-point drop in pH from your fully de-gassed water. Do this slowly over several days to allow fish to acclimate. Let a glass of tank water sit out for a few days to de-gas it. Make sure that the dKH of your tank water and the test water is identical and then compare the test water pH to the tanks’ pH about 2 hours after CO2 injection begins. Many of us go higher than a 1-point drop.
2) Excel is not needed if CO2 is injected at appropriate rates. It is good as an algaecide, but it would be better to try to first eliminate algae with good plant health. The H2O2 shouldn’t harm the plants if target dosing as you did but, again, better to focus on getting healthy plants, first.
Further information that would help:
- Light (make & model): ideally, PAR and PUR reading at the substrate? Is the 6 hours the total photoperiod or just at the highest setting?
- CO2 setup: is it pressurized or DIY? How is it timed with your photoperiod?
- Current PO4 and TDS readings and which test kits/devices are used for each of your tests?
- What you are dosing (product and quantity), in terms of ppm, and how often? This calculator may help:
https://rotalabutterfly.com/
- Substrate type and how long has it been in place?
- What is your filter setup?
- Cleaning regimen (filter and water change frequency and amount)?
- Circulation: surface rippling and are all plants gently moving from top to bottom?
- What is your water source and do you use a water softener?