Jenni that's a tough question to answer. I have my 2 36 watt PCs in an enclosed hood with reflector packed closely together over my 10. The hood results in the PCs being just about 2 inches above the surface of the water. Usually in larger setups, not only is the tank deeper, but the bulbs themselves are further above the water surface (which I imagine yours are), resulting in a pretty major dropoff in the amount of usable light entering the water. Sanjay Joshi, a major lighting expert in the industry, tested this with metal halide lighting, and found there is a major difference in the PAR put out in the water column when comparing lights mounted 6", 9", and 12" above the surface.
Answering "is this enough light?" questions is very tough, and particularly in this instance. I don't think there is anyhere in that tank though that would keep the clam for the reasons above. If you took those two 65 watt PCs, put them really close together, and put them right near the surface (as is the case generally in nanos) it MIGHT work. I strongly recommend you either upgrade or find another home for the clam. Believe me I hate generalizations more than anyone, like "Clams need metal halide", but in your case Jenni I think that's the best advice.
As tangsfornuttin mentions a DIY metal halide setup can be done for much cheaper than assembled metal halide fixtures. Just bite the bullet and do it. You will be MUCH happier in many respects with your metal halide lighting, and so will your pets.
Does that help? You deserve a
for looking into this, and you'll get another
if you make the right decision...
Johnny
PS- Also keep in mind Jenni that you chose one of the higher light clam species. Maxima (yours and mine), along with crocea (in reefnuts pic), are the two higher light species. I also have a squamosa on my sandbed that does fine, and I have seen those successfully kept (along with Tridacna derasa) on sandbeds of VHO lit tanks. If you ever upgrade your lighting try starting out with a squamosa...