Hello Kevin,
My tank is 4 foot. I have fish (7), coral (hard and soft) and live rock. i live in jhb so all is bought. 2 canister filters and a thing that takes the nitrite out of the water, a skimmer and 4 power pumps.
I dont think the "tick" is a crab because i cant see any legs or its head as a matter of fact. It is slow and although it is shaped like a big tick its rear end is slightly smaller than its front but still big and round like a tick with its underside flat. Around the outer edge of the underside it seems to have tiny bristles and it looks like it has armour not soft and smooth like a tick.