Just a thought on your question of powerheads.
I run a single 402 in my 27gl and it works great!
However I would go with a pair of Maxi Jet 1200's instead.
The Maxi Jets are smaller in overall size, push 25gl an hour more, and the baskets that fit to the bottom of these pumps are way way smaller than the larger 402 baskets(the baskets help prevent things from entering the intake of the pump). You get a better, smaller, easier hiding pump for $1 dollar more than the 402.
cheers, Rich