yeah it's normal. Tannic acid is released by your driftwood. It's perfectly normal and your fish actually like it. If it really bothers you you can remove the driftwood or take it out and treat it. I'm not sure exactly how to treat it but I believe it involves soaking it in water for like 3 months changing the water weekly. Not sure though. welcome to the forums. This forum is just for introducing yourself please post any questions about fish in the respective forum. If you are a beginner I doubt anyone will complain if you post in the beginners forum if your not sure where your question goes.