[Delphinus]

I agree with the compatibility points, that's why I've always run multiple tanks myself. At one point it was advantageous for me to have at least one system that was completely separated from the others, because of the potential for fish kill (I had a 24" diameter ritteri anemone which could kill off an entire tank full of fish if something annoyed it .. it would just release something that would cause the fish to die within minutes. Happened to me a few times and it was a terrible traumatic experience so I mitigated the risks by putting it in a tank by itself, a dedicated species setup.) I don't have the anemone anymore, sold it, but the legacy of the multiple tanks lives on.

Which brings me to the second point, 2 systems is twice the maintenance work, and so on. In the wintertime, when we're housebound anyhow, it might be less of a concern but in the summertime with vacations and outside experiences taking precendence it's easy to fall off the wagon and it's amazing how quick tanks can go from "A+" to "C-". In reefkeeping, if everything is OK, things can be amazingly low maintenance. But as soon as one thing gets off balance, it can be fairly easy for the house of cards to collapse.

But if you run two tanks off one sump, at least you can leverage the equipment factor. Instead of two so-so skimmers you can afford one really nice one, and so on. but then you're still stuck with the "all eggs in one basket" scenario.

But bigger systems buffer against problems better than smaller systems, so it might lessen the need for redundancy some.