Normal practice in feeding fish is to feed them as much as they can eat in five minutes. Any uneaten feed that remains after half an hour of feeding should be removed from the tank. When we feed smaller fish with smaller granules of fish feed, say less than 1mm, feed spreads over water quickly and it becomes hard to assess how much is uneaten and it gets even tricky to remove it.
I just tested with a one inch high ring of 4″ PVC pipe with a few pieces of thermocol aound it to keep it afloat. This keeps the feed inside the ring without spreading across the tank. Also this makes removal of any uneaten food easier. This may not work if you have bigger fish in the tank as they will shake up the water and start playing flying disc (frisbee) with the ring… 🙂