Hi Adele,
the symptoms you describe: hanging up by the heater but coming down to eat, the shabby, frail appearance and the flakiness around the mouth are all symptoms of aggression.
The fish who are hanging up at the top are doing so because one of your other fish is not allowing them to swim freely. You often have to watch the tank very closely and for some time AND without the fish knowing you're there to see how this is accomplished because quite often it is done without any kind of physical violence at all, although that may in fact be taking place at other times hence the shabby, frail appearance.
The fish at the top will start to come down and then you'll see one of your other fish, the aggressor, turn ever so slightly in his direction and that's all it takes to keep the fish cowering at the top.
Lip-lock fighting is likely causing the problem with their mouths.
This is not a situation that you should allow to continue. Injuries may become infected endangering all of your fish and it's just not a happy tank to be in or to look at when you've got fish that can't swim.
This is an 180 L tank, correct--? So that would be about 45 gallons which is considered small for mbuna, especially for the species you are keeping. I'm afraid you will continue to have aggression problems until you either change the stocking of your tank or get a much larger tank. First of all most mbuna need to be kept harem-style with 1 male to 3-4 females. In that size tank you need to choose one, maybe two species of mbuna.
So my advice to you is to decide which of the fish you now have that you'd like to 'build' your tank around.
Lombardoi: this is considered a High Aggressive fish. If you want to keep this one it should be the only species in the tank and you need to get the male/female ratio correct
Sprengerue--(Rusty). This is considered a 'Peaceful' mbuna. You could have a group of these with either the dems or caeruleus--
demasoni: these like to be in a large group--not sure how many you should have in your tank but someone else will hopefully comment
Caeruleus, (yellow lab) these are also considered peaceful. A breeding group of 1 male and 3-4 females would be great in your tank.
I think this is one of the hardest aspects of keeping these fish: the day they stop getting along and you have to decide who stays and who goes.
Please post back with any additional questions.
Robin