Croceas are fairly light demanding, however as they should be placed on the rockwork, where they will bore / attach, if you can get them within the top 8' or so, directly under the lights they should do very well, I wouldnt go any lower than that, however I had 2 in my 60g under 8x65pc one attached in the rocks, the other refused, and finally left him on the sand bed. The one on the rocks did extremely well, and the other did nearly as good (at 13" deep)in that tank for the past year+, now both have been moved to my new 100g under MH. For these clams you can tell if they are getting enough light or not, the "mouth" inhalent siphon shouldnt be gaping open, it should look relativiely zippered. If the clam is fully extended and mouth is gaping, It is not getting enough light, and is trying to maximze it's surface area.