fish questions

Question
i have 2 african ciclids in a 20 gallon tank. they have been together for 4 years nowmy question is this. my chiclids are 4 years old and very territorial. Are there any other breeds of fish that i can put in their tank? also recently in the morning when i turn their light goes on they look like they have locked their mouths/lips together and do this little dance.its strangeIt looks like they are sucking each oter in the mouth. Is this a sexual thing?? thankyou very much Patti

Answer
It would help to know what species of Africans you have however a general answer is as follows:  In a 20 gallon aquarium you could put 4 pair of medium Africans in it as long as you have plenty of hiding space and decorations to break up Territories.  In your situation your fish have established set Territories and putting any new fish in the tank would result in the new fish being beaten and possibly killed. The jaw locking your talking about is pure aggression and will eventually result in one of the two being seriously injured and unable to defend itself which in turn will eventually lead to its death.

Good luck,
Craig.