Don't target feed clams, it's completely unnecessary. They get all of their nitrogen through you feeding your fish, fish poo, etc... They can also take in N & P directly from the water, but they do require a lot of light. If you're not seeing new (white) shell growth at the upper margins of the shell, you simply are not providing enough light. A long time ago, some idiot said that smaller clams had to be fed in order to survive, this is scientifically false and has been studied several times. Clams can receive over 100% of their nutrition from light alone.