I just got a nice piece of drift wood from the fish store, and it's putting a lot of tannins in my water. I read that it is good for Neon Tetra and Catfish, but can cause ph to drop to low levels. Is a weekly water change of about 15 to 20 percent enough to keep it at safe levels? I dont mind the color, I just don't want to lose any fish. I have neon tetras, cory cats, hillstream loach, and mini mexican crayfish and will be getting red cherry shrimp very soon. Thanks for any info.