A 20 gallon with 12 electric yellow cichlids, plus a bumblebee cichlid? That's definitely overcrowded. Bumblebees can get to a decent size and are very aggressive. I speak from experience here. If I'm not mistaken, electric yellows are rather timid and stay rather small, although the exact size eludes me currently. In any case, twelve is far too many in a tank of such size. I would look at rehoming several of them, including the bumblebee.
You also said you sat the 20 gallon up only 3 days ago. That's not enough time for your tank to efficiently cycle. I would expect some large spikes in ammonia and nitrite, and I would begin doing large daily water changes. 50% would likely be sufficient, depending on how high your levels are. Do you have a test kit? If not, I'd recommend getting a liquid test kit, as they're much more accurate and last a lot longer than the strips.
Welcome to the hobby and the forum! We're all glad to help in any way we can.
