I agree it looks asmall 50 gallon but could be the camera perspective. As for crowding the tank, i agree with the others and would say your fish would probably be a bit happier with a bit of cover in the way of wood or plants.
heres a better pic. the tank looks bigger in person. it does seem small in the pic. There's no fish in there yet im planing on getting a couple today not sure what kind i dont want anymore a**hole fish. that maneguense distroyed that tank and ate everything in site.
I think thats a 20 gallon tank, you would think that there would be a lot more room left over with the rock in there, but it doesn't look like that much.