sand sifting star vs. hector goby vs. conch

[troni]

ive done lots of research on them but get to big... i've been hearing move to a larger tank waaaaay to much lately. i must be running into some money here soon!

[Cubeman]

I think the fighting conch is too big for the 20. I've got one in a 90 and the available sand bed barely seems big enough for it (they don't climb well so you need lot's of open space). I would support the Hector's Goby (or a Rainfords) as a good choice for your situation.