diz re: your firemouth
To proper address you original question....
which Im not sure was really answered...
"..help this fish...."yes.
Remove the Firemouth to the 10, which I assume u'v done.
Turn the lights off in the tank and add about 1 1/2 - 2 tbls spoons of salt.
Make sure the water is filtered and clean. Dont muck up the water with a bunch of stress coating and slime stuff, just use the salt - ( AQUARIUM SALT< ROCK SALT< PICKLING SALT> )
Watch for a days or so and see if it is swimming more.
If not, look for bloating or lesions. Consider a medication if you think there is an internal problem or wound.
Generally,, the trauma of being picked on by an other fish is alievated by putting the fish into a environment that is safe and clean, where the fish can be secure and recover.
As far as keeping the Firemouth in a ten gallon tank...
as Im sure youre aware (heh) its not recommended by most hobbists, BUT.... there is no reason you cant do it.
A cichlid, even one that grows to 6-9 inches, will adapt, much the way captive birds adapt to living in a small cage. Im not fond of that idea either, but whatever.
READON...
Providing you do what you can to make the animal happy, then you cannot be judged.
Personally, if I HAD to keep him in a 10 gal, then Id make sure he had a kewl cave to live in, and lots of plants, fake or real, ( it makes him feel safer). Id feed him the best food, and I'd make a pet of him. Id give him frozen foods like bloodworms and krill, and a decent pelleted food. Id pay attention to his water and its chemistry.
Perhaps when he got older, Id give him the occasional feeder. (Keeps his hunting skills intact.)
Id clean the tank as I should, and Id be proud of him.
If he's young now, perhaps larger cory could live him and he may accept it. Best to do it when he's young tho.
As far as his growth goes,
he will eventually begin to slow in growth as his tank gets smaller.
There are ways to contol the growth rate of your fish(pl.)
in a captive environement, but I am not going to be the one responsible for putting that notion in your head.
I think with your Kare-free ways, you could do more damage than good. (No real offense intended.)
So, back to the firemouth in the 10...
Eventually one of two things will happen ( at any time of course, but ...) he will either adapt and continue to live and be happy in the tank,
or he will
a.) become over aggressive and display abnormal and possibly self-destructing behaviours, or
b.) become chronically ill where you will spend $$ on medications.
(AND if ..."you dont kare...) I dont wanna hear it...and neither does anyone else. I just consider that display of disregard inhonorable and offensive. Keep it to yourself. Ask the questions, then accept the answers like a decent person. Thats all.
Just know that most times, but certainly not all times, you will have move him into a bigger tank.
Hope this helps.
Exodon