Change your water and while you are at it use a nut pick or knife and poke and prod the wood in the aquarium to see if you can find any rot, loose stuff and so on. Remove all you can, it is a pain but you may have to remove the wood and prepare it again for use.
Until you find the source of the organic material there really is no point continuing the PP treatments.
Before the water change
- Pour 25-ml of distilled, bottled, filtered water in a glass beaker.
- The idea being clean water without chlorine or dechlorinators.
- Draw 25-ml of aquarium water from beneath the surface and
- Place it in a clean glass beaker.
- Place 1-drop of the 0.1N PP solution in each beaker, swirl and cover, clean cloth will do.
- The precise amount is not critical, I am calculating the drop= 1∕20-ml.
The beaker with the distilled water is the control; the color should remain pretty much the same for at least 24-hours.
The beaker with the aquarium water is the test sample and the dose is approximately 6.3mg∕l the idea is to see how many drops are required to stay in the pink for 4-hours.