please help - i have a poorly fish...

hope someone can help me...i've just joined this site and am new to fishkeeping. one of my goldfish is looking really poorly - he's never grown as much as the others, but always seemed ok, but a day or so ago i found him lying at the bottom of the tank looking listless and thin. His gills are red (not at all pale), and there's nothing wierd on his body, and he's not scratching or anyting. When i added some food, he came up and ate some, but spat some out too, then he went back to the bottom of the tank...am really concerned now and dont know what to do. The following is a summary of the situation:

got tank 15 weeks ago - it measures 15 x 24 inches (18 inches deep), so about 100 litres
it has a 'Fluva' power filter 3
it now contains 2 blackmoors, 4 fancy fantail goldfish and a plec
i measure the water at least once a week, and its always within normal parameters, but the ammonia did go high the other week (1.2)
water at present is: pH 7, ammonia zero, nitrate 5mg/l (dont have a nitrite test kit yet)

what have we done to make our fishy so ill? we followed all the advice from our aqautic centre and not sure what to do.

all the other fish are fine, they've all grown well and seem fat and happy. One of the fish has a tendency to lose balance and float to the top of the tank, but i changed the food i was giving from granules to flake and have started to add some cooked peas occasionally and that has made it better (although he still does it sometimes - is this related to the problem with the other fish? - put some swimbladder treatement in the tank the other day...hope that was ok...)

hope someone can help ... i dont want him to die and its horrible seeing him so miserable, just hope he's not suffering.