Using a U/V is always good even for GoldFish. I have a 25 Watt U/V on my 150 (5x2x2) that is being used for 2 VATF'S, Spot Gar and 4" Motoro pup.
Add all the filters you can (more than 1 filter is always a plus) and the bigger the Sump the better. If you have a Sump (wet/Dry) thats where I have the U/V. Between the Sump and the 150 (powerhead in the Sump going to the U/V (U/V is NOT in the Water) and from there, directly to the 150 with a spray Bar).
Just incase you didn't know