Personally i would get the water parameters in a better position first and monitor. If things dont improve then consider aquarium salt, but i dont like treating in a main display tank. A small amount, 1 tablespoon per 20 litres wont hurt anything as a preventative measure, double this to treat serious infection.
As to filters, your filter is rated for your size of tank, but goldfish are very messy and you need to look at filters rated for double the tank size for them.
Pets at Home have a fish point system. They assign a tank a number of points, i think they also add extra points if the tank has a better filter (im not sure on that). And fish have a number of points also. If a tank holds 20 points, and a tetra is 1 point, your tank can accomodate 20 tetras. If a goldfish is 10 points, your tank can hold 2 goldfish. This is similar to the 1 inch of fish per gallon rule of thumb and works well with small fish, but fails with big fish in small tanks. A 100 point tank may well hold 10 goldfish, but a 10 point tank wont be big enough for 1 goldfish, if you get my point. The employee is only doing what they are told, they probably know less about keeping fish than you do.