Because it's an old myth that needs to die fast. It was once thought that small clams required feeding , which is completely false. Tridacna's have the ability to filter feed, but it's not the main source of their energy/food, they derive nutrition from their mantle, driven by light. This is a very simplistic explanation, but you get the idea. Clams do best in full spectrum light, but are extremely adaptive to different lighting conditions, as long as they get enough to drive growth. Both my maximas were small when I got them, probably 2 1/2" and my noae, like Orion's, was larger, probably 4"? To really appreciate their full color, use a "day" spectrum (6500k to 10k) and you'll see all of their colors as they should be. Don't be afraid of small clams, in fact, it's really rewarding to watch them grow. They key to buying a clam is looking for new shell growth and a nice full mantle, with no damage to the foot/byssal organ.
Clams are like sps, once you get bitten, you can't stop.
