I just read the thread from post one, all the way through. I love what you're doing with those tanks, and it's a great read with great tanks to look at. It's a brilliant set up.
Keyholes. They are such gentle fish, and good fish watching. If you ever have space and see Guianacara spp, consider them. They are similar looking, but take all the attributes of a keyhole and notch them up. They're actually Geophagines, and are a complex of species that are hard to tell apart. It's best if you know their origin to figure them out. I had one which bred, and was one of the most interesting Cichlids I've ever watched. Another species was territorial to the point of impossibility - really aggressive. The first had been more alert than keyholes, but not especially aggressive. So that is their drawback, but if you have space and encounter one of the more easy going ones, they are really something. No keyhole fan should avoid them.
A person could spend hours going from tank to tank in your set up.