I would say to be carful. The sand in of itself would not be harmful, you just have to be carful about introducing any unwanted pathogens with it. Also make sure there is nothing in the composition of the lake bed that would alter the chemistry of your water making it incompatable with the fish you are keeping. This most likly wouldn't be an issue with localy bred domestic strains but some newly imported wild cought species might have a problem with it. PFS runs about 10 bucks for 50 lbs and is claen safe to use.