hey if you really want to have mushroom and you want to use a 250watt of light... place the mushroom near the bottom of the tank and that might help.......... my 2cents.........
What kind of clam you intend to keep, maximas especially ultra grade ones....... MH with 250watt and above is recommended but for squamosa or lower light demanding clam 150 watt will do........... But if you really want to try out 150watt with ultra maxis.. put the clam near the top of the tank