I had an 100l tank with a skimmer on it, and had it for my daughter as a project. The only fish in it was an Emperor Angel Fish, but for some reason it decided to spend Saturday afternoon evening dying
so on Sunday, we popped across to the rock pools to get some rock fish to keep the cycle correct, and during our hunt, a tenticle swooped out from under a very small ledge at the rock fish we were hunting... long story short, we came home with a 30cm octopus, that is from head to tail, so to speak.
Kids rapped, and all looking well at the moment.
There is nothing else in the tank except dead coral and maybe/maybe not a rock fish.... might have become breakfast..