Hey Whedbee, sps are doing good at 4 inches off the sand, lights are 4-5 inches off the water.. soo 2 ft?
its all open. I do have a middle section that has a 1/2" glass brace covering it (bout a foot wide?) and notice very little reduction in light penetration.
Clam would probably be fine in a 21" tank. i've never kept a clam so I can't say.. if you have a par meter you're welcome to come over and check the level if you want
