chichlids not eating

Question
I have a 100 gallon tank with two emperior 400 filters that have been going for 1 and half years now.  I have two large jack dempsies, one new small green terror, one electric yellow, and one mixed.  I change one third water bout every two weeks.  k, the jacks haven't been eating much for 4 weeks now and 2 weeks ago the mixed fish keeps standing on his tail.  I feed mostly Hikari gold in mini size but i also give them a little zucchini and some corn every once and a while.  the jacks also seem to be swollen at the stomach and their back half of the fish seem to be a little thinner than they should.  is this constipation, and if so what should I do.

Answer
Hi Jason;

It could be an intestinal parasite of some kind. Or, you could be right and it's constipation. It's also possible they have swallowed pebbles and they can't pass them through their systems. It seems strange that they would both have these symptoms at the same time though. You might contact Ron Coleman and see what he thinks. He is a cichlid expert. Here's his profile page;

http://www.allexperts.com/ep/1472-21527/Fish/Ron-Coleman.htm

At Your Service;
Chris Robbins