I'm afraid no clams is the best answer. The smaller clame (which would still get WAY too big for a 2.5 gallon require much more intense light than that. THe lower light clams are too big.
Even if you got a small clam that would be fine with the lights, they place such extreme chemical demands on such a small system and also demand a stability of tank that's extremely hard to keep.
If you are deadset on a clam, consider at least 10g+ with MH for a small maxima or crocea clam. A larger tank with lesser lights would work for a squamosa or derasa.