I have had this species for seven years now, so hopefully I can offer some advice. The citations in the previous post are generally accurate, though with some cautious reservations concerning tankmates.
First, let me say that you will absolutely never see this fish, ever. If you "train" it with frozen bloodworms, you will see them at feeding time but never aside from this. "Nocturnal" might have been invented for this species. They will only venture out in complete darkness (meaning the room as well as tank), aside from the feeding. I trained mine with frozen bloodworms to come out in the evening on water change day, and they sit waiting for this, but they will not venture out until the food appears. This is not a good basic food, and as this fish feeds from the surface, adding some floating pellet-type food well after lights are out is the best way, with a bloodworm treat once or perhaps twice weekly.
Second point, they must have wood in which to live. Their common name, wood cat, is certainly well justified. They reside in tunnels in bogwood; the Malaysian Driftwood available in many fish stores and online is ideal, Get some pieces that will stand upright, representing tree trunks, and that have tunnels. The fish will select their own "home." However, this wood must be standing close to the filter return so there is a relatively good current hitting the wood. When I first acquired my three, they took up residence in two standing pieces of wood, at the end next the filter return. A few months later, I moved them into a larger tank I set up, and I moved them in the wood which was easier than trying to dislodge them which could cause damage. I decided where the wood should stand in my larger tank, and carried it in a pail of tank water to the new tank and placed it. One piece (which happened to have one fish in it) I placed at the end next to the filter return, and the other (holding two fish) went at the opposite end of the tank. Next morning, I discovered the two in the one piece had moved to the piece under the filter return, and all three were now residing there. They need water current.
Now to the caution on tankmates. These fish will nip fins. I had a group of Rosy Tetra, and noticed one day that two of them were missing the lower lobe of their caudal (tail) fin; with weeks of observation I saw no nipping from the other fish (this is a large 115g tank, 5-feet in length, so plenty of space). I considered a bacterial/fungus issue, and consulted a microbiologist. She saw the photos, and as soon as I mentioned the woodcats, suggested they were likely the culprits. This was four years ago, and having no other tanks I left them and kept an eye on things. One other tetra lost a bit of its tail since, but no others. You are intending these fish in a 20g, so I would suggest no other fish as there isn't much room. The woodcats are quite rapid swimmers when they come out in the dark; a couple nights early on I observed them with a flashlight around 2 and 3 in the morning. I also think the feeding issue is part of this; placing food in the tank after darkness is fine, but other fish are nocturnal or semi-nocturnal too, and the fin nipping might have been due to lack of sufficient food. I'm now feeding more often, about five hours after the room is in darkness.
Feel free to ask questions.
/www.seriouslyfish.com/species/centromochlus-perugiae/
