Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The finer the gravel the better. I have used gravel from almost 1/2" diameter (way too big) on down to sand.
The best size seems to be 1/8" or maybe a bit larger, then on down to sand. Anything over about 1/4" is a bit too coarse.
If the used substrate has been sitting dry for a while, then there is almost no possibility of fish diseases or parasites. If it is fresh from a healthy tank, then almost no possibility of fish diseases or parasites. However, if there is any chance of transmitting a problem I would solarize the gravel:
Dampen the material, then put it in a box with a glass lid in the sun. The shallower you can spread out the substrate the better. Move the box every few hours so the sun enters it at 90 degrees. You could put an oven thermometer in there to see how hot it is getting. Even on a cold winter day, a clear sunny day, out of the wind, will be enough to sterilize the substrate.