From the photo...what are the light specs? There may be an issue with spectrum.
I am not a fan of these "soils" as plants do not need them, but I won't go into that as it is in the tank. But do not use buffers, pH adjusters, or similar. These things do weaken fish as they get inside them, and while they may not (but they can) kill the fish, they are weakening the fish and making life more difficult, and stressful.
I checked the fish species, and you have some issues here, I will get to that below. With some adjustments, the lower pH and zero KH will be ideal for the very soft water species. My tanks run zero GH/KH and the pH in some is well below 5. BTW, what is the GH?
The overstocking is not so much numbers as combination of fish that really should not be together. This always impacts on the biology, likely stresses some of the fish, and here again lads to other problems.
I would remove or not replace the Florida Flag Fish. They really need a larger space.
The CPD I understand, but they would be better with a larger group (20 or more) in a 20g tank [more data here:
Celestichthys margaritatus – Celestial Pearl ‘Danio’ (Danio margaritatus) — Seriously Fish ].
Second filter does not help as the tank size and the species requirements (numbers, etc) and their compatibility all factor in, and this has nothing to do with filtration. This is why stocking sites are so near-useless, they cannot possibly factor all that really matters to fish health.
I would up the W/C to 50% minimum, once a week. If you want to know why, check out my article which was just posted today. Pleased to answer any questions I can.