Tridacna Deresa
Per ezcompany
These clams are probably the most forgiving in terms of light than the rest of the mentioned clams. Their maximum depth of occurrence is about 25 meters (J.F.), but like the Gigas, they will be much happier with the lighting requirement given to Squamosas.
Exceptions to the Rule
Many people claim that Croceas and Maximas can survive under power compact lighting. These cases are far and few between, but there are certain individuals that are able to tolerate lower lightings, at which most of their counterparts would not be able to survive. Nevertheless, you should not take the risk and hope to get lucky, it is better to be prepared to give them what they will surely thrive in. It is always good to provide at least enough light of the given species to thrive in, and NOT the bare minimum you think you can get by with. Plus, it is the least we can do for our animal, as well as our responsibility as hobbyist.
Light Acclimation
There is no artificial lighting stronger than the great ball of fire in our sky, so all clams can adjust to your lighting provided they are given proper acclimation. This is especially important to prevent light shock, and gives time for the natural population of the clam’s zooxanthellae to adapt to their new habitat and photo conditions. Acclimation can easily be done using egg crate screening, or other materials that cut/diffuse lighting intensity. Remember, switching from fluorescents to metal halides is a big change, and so is switching from 150 watts to 250 watts.
Bulb Life
Fluorescent light bulbs need to be changed about every 10 months as they lose their intensity from old age. For metal halides, people have reported that 20ks lose their intensity at 6 months, 14ks around 8-10 months, and 10ks almost one year. I have not seen solid evidence in regards to this matter, but personally change my 14ks every 10 months.