If I were to setup a 72g tank with fish, LR a few soft corralls and a couple of clams, how much and what ttype of lighting would I need? Would 220W be enough? I believe the tank is either 22 or 24" deep. Is there a general watts per gallon rule?
I love halides but VHO very close to the water and plenty of them will be fine for low kight clams like Derasa and Squamosa. they are not really low light but will tolerate lower levels than Maximas and Crocea.
I have 2 175 14K over a 75 and I think thats plenty. I would recommend an electronic ballast no matter what you get for efficiency and longer bulb life.
It is posible to keep clams under PC and t-5 lighting as long as they are placed towards the top. Now the growth may not be a fast as having them under MH but they will do fine. I have a maxima under 96 watts of PC for over 4 months now and doing good. Remember that Light is not the only thing clams need. So please make sure that you research the proper care for clams.
I have a Crocea and TearDrop Maxima in my 75gal for the last 18 months. I am running 4-110w VHO's. The Crocea is on the substrate at center tank (twice the size as when I introduced him) and the Maxima at the top center. I have also kept Derasa's under this lighting. Have no complaints.
I have done all teh haides except DE bulbs. I think that in a tank less than 24 inches 175's are great and VHO's can be be great as well. for me its the look I like. But do some research and you will find people keeping all sorts of things alive and healthy under T5 and VHO
The best thing to keep in mind is that clams come from very shallow water in most cases. With exception of the gigas and hippopus as far as the species we deal with. Unless kept at the very top they will not get the light intensity they need.
Metal halides do provide the intensity needed to keep clams happy. They also need to be provided with some source of phytoplankton. Don't forget that.
The lighting you speak of I feel is not intense enough for long term clam care. Especially with that deep of a tank.