Thanks again for the information.
What I'm trying to do is find out what's been done with the water as this is where the problem lies.
It's most obvious that the problem lies within the main tank, as you have mentioned when the fish are moved to another tank the problem disappears.
So the first and most fundamental thing to do is have the water tested for levels of ammonia, nitrites, nitrates and ph. Ideal test results should be ammonia and nitrite=-0-, nitrates=20 ppm or lower and as this is a cichlid tank (I'm assuming african cichlids) the ph= 7.6-8.4.
The easiest way to test is to bring a water sample to your local pet store where they will usually test for free. If any of the test results are different from the levels mentioned above then appropriate action should be taken.
If the problem lies within the levels of ammonia, nitrites and/or nitrates then daily 25% water changes will be necessary. If the ph is too low or too high than it would need to be adjusted and stabilized.
Keep in mind that if the fish were suffering from a disease their behavior would not change if moved to a different tank which is why the problem definetely lies within the water quality of the tank in question.
It's also possible that there is a problem with heavy metals present in the water for which a product called Chemi-Pure, see link below, added to the filter media will help.
I would also recommend that you check all aquarium decorations, ie., rocks, fake plants etc. etc. as there is a possibility that they are leeching harmful substances into the water thus tainting the water quality.
So please follow the above recommendations and let me know if you encounter any problems.