I did at one point. I'll see if I can refind the paper I found, but it was out of the Transactions of the American Fisheries Society if I remember correctly. The general findings were that while they did on occasion eat somewhat larger things (one had like a 4 inch fish in its gut), they generally focused on the tiniest of tiny phytoplankton. When I compared the sizes of food particles they listed in the paper to hook sizes, the average was around 1/10 the size of a #32 hook. The gill rakers on those things look like a sponge. It's amazing.

Honestly, when the food gets that small, I don't think they're actively chasing it. The energy cost would be enormous in comparison to what they gained from it. So, if we could drift a teeny fly in front of a filter feeding fish, they might take it as part of the cloud of food they're feeding on.