When you do a large water change try doing one really big, then take a spray bottle with Excel and hit the wood with it.
Then refill a minute or two later, this will kill any BBA on the wood.
While the tank refills, take a hard toothbrush and you should be able to get any of the BBA left when you could not spray.
Make sure not to get any excel on plants, it will burn them at high cocentrations.
Likely might need to clean the filter more often, and focus on good current at the surface/mixing and then......CO2 slow and progressively and watch the fish.
They will turn dark and act lazy, not feed etc if there is too much
If they are new, they do this anyway........so you have to be careful before deciding there.
I'd do Excel daily at 90 mls for 2-3 weeks to beat back any BBA as soon as you do the water change and attack it aggressively.
This should beat it way back and help the issue.
CO2 and decent O2 from there.
Most soil declines in Nitrogen only over time, 6-18 months typically..........
So if you dosed say Traces/K+ before and now dose other things......you might just add some KNO3..........
