Ros, are you willing to invest a little more to have a great display tank? I'm partial to 70 gallon tanks. If you purchased one more 250w MH DE and place the bulb at least 10 to 12 inches off the water, you'll be on track for an excellant display tank. A 30 gallon fug with a Mag 9 return, some Chaetamorpha, a fug lights, a titanium heater in your fug, an internal or external overflow, a couple of powerheads for circulation, surface agitation/ gas exchange and a few other pieces or hardware and you'll be heading in the right direction. Of course you can go smaller as Snakebyt suggested. A 58 gallon would be very nice too, but you're only 12 gallons away from a killer tank and more room for zoos. No offense to snakebyt, just my opinion is all. Good luck.
Mucho