You can add a small CUC when you get your first fish but make sure you quarantine that fish first. I setup and cycled my QT and DT at the same time, so both tanks was ready in about 3 weeks and then I add put two clowns and a firefish to QT while I give more time to the DT to be ready. Then I put a couple of snails and hermit crap to start working on some diatom that start growing on my DT and they did a great job on it. Everything when good and I have those fish already on my DT. I add later a emerald crab that is amazing how it clean.Thanks will do. Add the one fish before CUC?