I switched my Fish Food from OSI to PennPlax.
My scum went away.
And here's a positive review for PennPlax fish food. For being the dollar-store name in fish aquarium equipment, the food is actually pretty decent.
The container has this built-in measured dispenser.
Open the container, pour some food into the dispenser, close the container, and slide the dispenser door open. No need to touch it...
and you always feed the same amount.