There's a few types that'd go in a 4x2x2, but be mindful that most of what I posted there are in the ballpark of several hundred dollars to several thousand dollars each - there's only really 3 that aren't above that with regularity. Most are also from deeper water and necessarily need cooler systems, tropical is not suitable for them and generally will cause them a very early demise.Don't you worry, I've been doing the power costs maths in my head of 4x2x2 tank again...would have to investigate whether I could transfer current equipment to it...meaning the same power costs as I have now..eg skimmer heater etc