I inherited a 10g tank from my daughter. I have 3 zebra platys and 3 orange mollies. I needed to change the gravel because of a problem with small snails. There were A LOT of them. My daughter brought one home from the store with a fish and the rest is snail history. I changed the gravel by scooping it out and then put the new rinsed gravel in the tank. I put some of the old gravel in a couple of nylons and kept it in the tank (hopefully no snails!) My question is how much of the old gravel do I need to keep in the tank. Will a week be long enough. thanks for the help!