Feeding will help your BTA's grow quicker. If that's something you want - then go for it. If you are trying to control their size a little better, they are photosynthetic and don't actually need to be fed.
Also, many here on the boards have had otherwise healthy anemones die after feeding whole silversides. In addition to them just being too big for healthy digestion, there has been some speculation to the quality.
IMO, if you want to feed, it's best to feed a mix of blended meaty foods that you would also feed your fish. It's pretty easy to chop up some clams, fish, squid, etc. and freed in a ziploc