i currently have about 40lb of gravel in my 46gal tank. they didnt have enough live sand at the petsmart to fill up my tank so i just decided to get gravel(back in november)..is possible to add sand into the tank and at the same time remove the gravel with out messing up the biological filter?.. my guess is add a little bit of live sand and remove about 5lbs of gravel each time. is there any other way you guys would prefer?.. and is it ok the add the sand while the fish are in the tank?..