I've been looking at quite a few posts on here and I can't tell if this creature that appeared in my tank is a sponge or not. It's a different color than all the other ones I've seen, because it's red with white frills, and it doesn't match the description of location on the common tank hitchhikers thread, being in a fairly low flow, non-shaded area of the tank. Here's a little video I took of it, too, where it can be seen in motion. It's pretty active for how low flow the area of the tank really is.