You will have to feed them live phyto pc lights are not great for clams
they are the lowest par you can get and I have had only average health,color and growth under them,my t5s and vho Mh do much better.
You need to keep the calcium up around 450ish for growth.
The reason PC gets such a bad rap is that 99.9% of the fixtures out there with PC bulbs have 100% worthless reflectors.
I had a derassa growing like wildfire in a 15 gallon tank with 2*55W PC, but I had AH-Supply parabolic reflectors that gave me 3X the light output then a normal fixture when measured with my light meter.
Get a clam that is at least 3 inches IMHO, they have a better shot at survival and you don't need to worry about feeding so much.