he needs a bigger tank, and he will get bigger, males reach around 10 inches and females 12.. so for one turtle, a 55 gallon at least... from what i understand, goldfish can carry more pathogens and bacteria than other feeders.. plus they add a ton of ammonia to the water.. how deep do you have the turtles water right now? it should be as much water as you can put in there without him being able to get out.
the petstore gave you bad information as far as lighting goes.. glass and even small mesh will filter out the uv rays he needs.. i only feed my adult turtles every other day (once a day).. but thats something that im sure varies from person to person.. as far as what to feed him? i like the nutramax turtle pellets, but i feed them a variety of pelleted food, along with frozen or freezedried krill as a treat, and anacharis (a plant you can buy at petstores).. most pet stores sell ghost shrimp as feeders.. try a petstore that sells saltwater fish.. they usually have them.. turtles are horrible pigs and will eat more than they need.. i usually feed them whatever they will eat in 5 minutes.. but that is again a preferential thing.. for my bigger turtles i put them in a separate container to feed to cut back on the mess in thier water.. i use a rubbermaid storage container with about 5 inches of water.. put them in and let them eat for a bit, then wait 30-45 min and put them back in the tank.. the mutramax pellets have calcium in them.. and i usually look at the protien content as well.
as far as why they gave you crappy info... i have worked at petstores for almost 6 1/2 years now and in my experience the majority of my coworkers either didnt know any better, or just didnt care enough to provide proper info.. maybe they were just trying to make a sale, or maybe they really dont know.. either way, its always a good idea to do your own research before purchasing a reptile.. did they tell you how long your turtle can live? im not positive about thier life span but i know it is something like 40 years.. or how big he will get? its very frustrating to me to work somewhere that sells turtles precisely because of this.. if you are really bothered by this, confront the person who sold it to you or speak to the manager and tell them that you felt you recieved improper information.. it might not help you out, but maybe (if its a good petstore) they will encourage the employees to provide more accurate info.. then this wont happen to anyone else.. sadly, sometimes they are just in it for the money and wont correct the employees.. but thats just how it goes sometimes..
i have to go to work now, but if someone hasnt given you a good care sheet by the time i get home, i will hunt one down and post it.. the pic is of the turtle food i use as the majority of thier diet.... ive never had any problems with it..
at any rate, i congratulate you on trying to correct the situation.. im sure the turtle will appreciate it very much also.. if you dont feel that you can provide the right size tank and long term care, see if the petstore will take him back. you were definitely mislead in what care he requires and IMO it is a valid reason to return him if you cant provide that care..

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my pets: clown treefrogs, reed frogs, big eyed treefrogs, tiger leg monkey frogs, gray treefrogs, milk frogs, cuban treefrogs, whites tree frog, green treefrogs, squirrel treefrogs, blue webbed gliding treefrog, chameleon treefrogs?, dusky salamanders, tiger salamander, veiled chameleon, box turtles, mud turtle, map turtle, yellowbelly slider, florida softshell, two saltwater tanks, four cats, two chinchillas, and a boyfriend.