I have a diamond goby (Valenciennea puellaris) that does a fantastic job of keeping the sand white. Mine has stayed small and tends to spit out the sand just above the bed rather than carrying it up into the water column like other gobies. The goby can get up to 6" but mine hasn't grown over 3", you may have to move it to a larger tank down the road if it does get big. They are common in Edmonton but call around for prices, the same fish ranged in price from $20 to $60 depending on the store. They will also eat regular fish food. Mine was shy and didn't do much for 2 weeks but now he is always cleaning and never hides.

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!

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.