depends on how think you want it to be. I am putting about 120lbs in my 125 it will be about 1 1/2" think maybe a little more. so I would say about 175 to be safe. Its better to have more then not enough
I would personally say atleast 200lbs. Depending on the look and personal preference of course. I purchased a 100lb bag of silica sand from Home Depot for $10-15 pretty cheap.