The easiest clam to care for would probably be the Hippos, Derasa & Squamosa, but with your lighting, (125g is not too deep) you should have no problems with either the Maxima or Crocea species. If you purchase clams that are 3" or larger, you do not need to feed them as often. most of their food source will be converted from their intake of the necessary lighting. I do still feed my clams twice a week though with a live phytoplankton product, but many successful clam keepers with larger clams never feed their clams and they do well and grow fast. You can culture phyto or just buy it in a bottle.
What you need to also do is to check your parameters in regards to calcium and alkalinity. You should be in the 420-450 range for Ca and 8-10 KH for alk. Minimal nitrates & phosphates and I assume your ammonia & nitrites are undetectable.