Fishy Companion For My Crayfish

Question
Hello!
I have had Dr. Blob, an Electric Blue Crayfish (Procambarus alleni), for a little
more then a year now. She's always been very healthy (molting monthly, etc.)
and has grown to be about 4 inches. When I bought Dr. Blob, the people at
the fish store recommended I get some feeder fish as well, so I bought 8 fish.
There are 4 adult fish left and the other day I discovered 3 babies as well. I
think they're White Cloud Mountain Minnows, but I don't know for sure; they
were only described as feeder fish. It seems like she isn't much interested in
eating them as long as I feed her regularly. The only times she seems to have
eaten any is when I've been away and there was no one to feed her, and she
hasn't eaten any since March. She also hasn't eaten any of the babies since I
noticed them a week or so ago.
I've always wanted to get another fish, but have held off because the bad rep
crayfish have for eating their tank-mates. However, based on Dr. Blob's good
behavior, I'm starting to reconsider. I've done some research and have found a
number of success stories. It seems that my best bet would be a fast moving
top or middle dweller that isn't small enough to be easy pray, or big enough
to attack my cray. Optimally, I would like a colorful, interesting fish. I really
enjoy sharks, and I know there are several types suitable for aquariums.
I was wondering if you could suggest some species that fit the bill?
I currently have a 20 gallon tank with standard aquarium gravel. It's equipped
with a standard filter, a bubble wall, and a heater (though I don't really use it).
The temperature generally runs in the lower 70's. I have some rock caves for
Dr. Blob to hide in and a couple plastic plants. I am also considering
upgrading to a larger tank.
Thanks for any advice you have!
Sarah

Answer
Hi Sarah,
Any small fish will automatically be prey.  I would choose an agressive fish only, such as any type of cichlids that do not get very large.  Cichlids such as Lake Malawi types.  Some sharks that are sold in pet stores only reach a maximum of 6 inchs.  I do not know what sharks there are at your local fish store, so I would research on the max size they get and go from there.  However there is still a chance that they will still get eaten if you buy them as a small size.
-Matt-