I'm not putting a full substrate. Probably only a single grain thick so some black liner comes through. Eventually the giraffe cat will go in there as well. I'm not that worried about gravel vac, as it won't be deep enough to really hold anything. It will probably look very similar to mixing with black sand on the black bottom of the pond. I've never used it, so was wanting to make sure I wasn't making a huge mistake before I put it in there. Sand is really my only option as the catfish have a habit of ingesting rocks and gravel from stories I've seen.