you might be able to get by with a BTA, but understand that when an anemone has to struggle for lighting they tend to inflate and can become huge. For example I have a LTA that was under my MH. When my MH light was going out and losing its intensity my anemone got be over 8in wide and very puffed out. It was trying to get as much light as it possibly can.
When I changed bulbs it shrank back to its normal 6 in stringy self.
I do have mine in a 29g. But its a streatch. In a 20 gallon, that anemone will sting anything it touches. I need to move mine out of my 29g or get rid of it, because I have a pretty full reef tank and I am constantly watching my anemone sting other corals.