Hi! I got 8 clams in my 2 x 2 x 2 feet tank! I use MH single 150 W 10 K bulb. I put my lights about 3 inches from the water surface. I switch on the lights for 10 hours and every 2 to 3 days feed them with DT Phytoplanktons. When I used PL lights, they did not open much but after upgrading to MH, they always open their mantles big big! My biggest clam is about a feet long and smallest about 2 inches!