Millipede is right! Theme tanks are real cute..... and can still look natural. How a bout a sunken aztec city with small SA spoecies to swim around it?
Or fallen Atlantis with smaller salties (or salty look alikes)?
Or mangrove bog with those fake roots from petsmart (made fish safe by filling the mold and filing suspicious holes), let it become murky abnd blackwater and put colorful low ph
stuff..or opposite brackish fish like mollies.
Sunken treasure ship with shark shaped fish including cleaner cats. Or tiger botia.! an 18 is going to keep you down to about 15-18 fish depending on sizes and tempermemnts. So run with the decor!