Squamosa,derasea,croecea, of course the maxima too. They are long lasters but don't skimp on the lighting! 150 watt metal halide at least.If you can provide enough light and good water quality for it then yes you can keep a giant clam (Tridacna maxima) in a 20g tank for a while. However, after 10 years or so it might need to be put in a bigger tank. They are slow growers but can get big if you keep them alive.
Any idea how long they take to get that big ? I'd like a big clam in my tank.....Squamosa, Gigas, and Maxxima get HUGE! Clear over a foot in shell size in their old age