People have had success feeding dwarfs frozen food, but the horses never seem to stay on the frozen food for long, and often they are horses within the group that never are weaned onto it. Frozen food can not be considered anything but a "nice to supplement with" type thing for dwarfs.
Everyone describes dwarfs as being extremely lazy. They like their food to moszy on over to them -- then they will take a bite. So you need living food that will swim right over. You need to get them some brine shrimp eggs (either decapped ones, or eggs that you will decap yourself) and start hatching them out. Otherwise, you'll end up having to put so much frozen food in their tank that the ammonia level will go through the roof.