you should get the biggest one grossly oversized for your application you can get, for sure. anything correctly sized or undersized might not perform as well
the bigger ones can be turned on/off but youll have that reserve power as needed. these are hotly debated items but given their abilities (sized correctly) against pelagic tank invaders id certainly have one if I had a large tank build coming up. for sure I would, and it would be a uv sterilizer for a pond not for an aquarium (did that in the 90s, worked amazing)
id find the biggest baddest one for the space id offer to mount it, and overshoot things. slightly slower pass-through is another great amp as well