Automatic feeding suggestions needed (marine)

Hi. I'm going to be out of town from the middle of next week until the
middle of the following week, about 7 days. I have a petsitter who comes in
to take care of my cats and other stuff, but they only come once a day.

To feed my marine (fish only) tank, I purchased a Penn-Plax "Daily Double"
which feeds twice a day. Unfortunately, the humidity coming off of the top
of the tank where it is mounted is causing the food in it to cake up and jam
the feeder.

Does anyone know a way I can avoid this with this unit? If not, does anyone
know of a better way to feed them while away? I have used the
dissolving-type food cubes in freshwater tanks before - do they make
something like this suitable for my clowns, damsels and hermit crabs?