The filters are to small there haven't been coping with all the fish in there.
Would help alot if you gave the size of the tank.
Overstocked for that filter seen as you won't answer the tank size.
For now I would invest in a bigger and better filter for the stock.
It still overstocked.
I would of bought another 205 to cope with all the fish.
You are going to have to get about 200 gal for all them fish, that if they survive in the bad water quality at the moment.
Don't think that smaller filter going to do much to be honest, only thing I can think of is rehoming some of the larger fish.
Which will take the pressure off.
/www.thetropicaltank.co.uk/rev-cond.htm to find a water conditioner that will help with ammonia. Some tests will still test positive for ammonia as they detect ammonium the same as ammonia. Ammo-Lock and other products work by converting ammonia to ammonium, which is harmless to fish but still used the same as ammonia by your bio filtration. A two reagent test will not give a false reading with ammonium.Got to do something fast as they just going to die of ammonia poisoning.
Good luck.
API Stresscoat & water conditioner does nothing to neuteralize ammonia and will actually raise the ammo level if you have chloramines in the water. Check out http/www.thetropicaltank.co.uk/rev-cond.htm to find a water conditioner that will help with ammonia. Some tests will still test positive for ammonia as they detect ammonium the same as ammonia. Ammo-Lock and other products work by converting ammonia to ammonium, which is harmless to fish but still used the same as ammonia by your bio filtration. A two reagent test will not give a false reading with ammonium.
Rehoming some of the fish would be best, as has already been stated. You should be changing water a lot more often than once monthly with that stocking, 50% 2 or 3 times weekly is needed.