Manually remove excess organics in the tank by gravel vacuuming and cleaning filter media in old tank water every water change.
Manually remove any decaying or dead plant matter.
Increase water change frequency, and the amount of water changed.
Consider spot treating badly affected areas or dipping plants / hardscape in a Flourish Excel, Met14 or H2O2 + water solution. Google search which method you think would work well, and for general ratios to mix a safe solution. Certain plants can’t tolerate these chemicals, so ensure you do a little research prior to dipping / spot treating plants.
If using CO2, ensure CO2 is dropping the pH of the tank water a full 1.0 – 1.2. To do this, measure the pH of tank water with no CO2 dissolved in it, and then measure again 2-3 hours after CO2 has been running. Ensure the drop in pH is a full 1.0-1.2. If the drop is not there yet, slowly up CO2 over a few weeks until at least a 1.0 drop is achieved, and watch fish / livestock carefully. Adjust CO2 down if you notice fish gasping at the surface and consider running an airstone at night when pushing a 1.2 or greater drop. For example, a tank water pH of 7.5 with no CO2 dissolved in it, should reach a pH of 6.5 – 6.3 for CO2 to really shine, and for maximum plant health.
Consistency in CO2 levels is key to plant health. Keep CO2 levels as stable as possible once a desirable level has been reached.
Thanks for all of your help hopefully I can get it under control