Clams in a 20L

A Squamosa, or Derasa would do ok, if you can keep your salinity, calcium, and ph all very stable within the limits that are best for the clams.

With this type of lighting I would pass on Croceas & Maximas as they are a little tougher regarding light issues.

Best of luck, Rob
I wouldn't keep any clam under anything but MH lighting but a few people suggest derasas are ok for VHOs but almost noone would recommentd clams under PCs.
I had kept 2 maxima's under 2 36 or 55 wt pc's for a year and the doubled in size in a 20l. they were about 3 inch under water level 5 in from the bulbs. then upgraded to a 120 and 4 110 vho and 2 400wt mh. bought a crocea from FF and got that lovely disease that killed all three in 1.5 weeks....
its possible to keep them in the 20 and pc's but keep calcium up and system stable....