Change in light intensity sometime cause clam to partially bleached, especially when they got stressed with change in temperature. They will recover without problem.
Evaboration is the price of keeping the temperature down, you may as well suck it up and take it like a reefer. Think of it as way to add Ca (as in Kalk addition to the tank).
colourmop,
12 g with a 150 W is a little much. I am not sure if you can keep the temperature down. Why get the 12 g, just get a 24 g tank. 12 g is so hard to keep because it is much more dificult to keep it stable.