There may have been additives int he plywood fouling the water like lamination glue or tree sap. Sounds like you did the right thing by re-working the setup. Curious to see how it goes!
I have a smaller tropical aquarium with an overcrowding issue. The darned fish (Tetras) keep breeding. Emperor tetras and red-faced tetras cannot stop themselves. It isn't even ideal water conditions being on the basic side. Just a lot of plants seems to make them happy.
