My Automatic feeder drops fish food into the top entry hole of a aquarium
via a 'plastic funnel'.

The inside of the funnel must be getting quite moist from the rising water
vapour, because fish food keeps sticking to the inside and then builds up;
to the extent I have to clean it every few days. So I think not much food
would actually get to the fish if I went away for a week or two.

Are there any nifty tricks to prevent the food sticking to the inside of the
funnel? Thanks.