silversides are fine.. but I wouldnt call them the 'best' they are more like the most 'common' anemone food.
For a sick anemone start with very small pieces of meat, like the size of a mysis shrimp. Even though the anemone can take larger pieces, they seem to respond better to smaller pieces. Think about when you arent feeling good.. you eat some soup, some crackers.. a little here and there. You should give him a variety (whether he is sick or not).
Silversides, mysis, any prepackaged fish store 'cubes' along with whatever you find at your local grocery store's seafood counter.. shrimp, scallops, swordfish, shark.. whatever they have! Just buy small quantities, its much cheaper than buying it from the LFS. Not to mention that silversides smell worse than anything Ive ever bought. I still offer them, but there are alot of other options.
Also, soaking in selcon or Zoe is a MUST with a sick anemone. I still do it for non-sick ones.
Good Luck! It takes a good few months to see improvement but, keep us updated!
Jess