I have a 20 gallon high with a piece of driftwood. It was actually a piece I found in a local stream. I've had it in the tank for almost a year now. I have some java moss and Java fern in the tank as well. But the moss and ferns are always covered with this brown coating of what looks like like brown dust. The reason I've narrowed it down to the driftwood is cause I have 3 other tanks that don't have this problem =( Soo I removed the driftwood. I'm going to shake out the plants a wee bit and than do a huge water change. Maybe smaller ones for a few days until it clears up. ... but you guys thinks it's the driftwood, right?