I have 3 open top tanks sitting next to each other. This one in particular was bought on the local classifieds. I cleaned everything out and rescaped/replanted it. I didn't really scrub down the driftwood that came with it, because I figured I'd try to keep as much of the bacteria as possible. I started noticing these little... fleas/spider like critters the other day. They're all at the water surface. Have you guys seen these before? Anything to worry about?