...it sounds like the pet shop dude just wants to get your money, because what he told you is [rude words go HERE]. There is no way your system is somehow ~~magically different~~ and needs $$$ expensive additions that nobody else has to use!
First, go to a different shop and get your own test kit! You need to be able to test your own water so that you know what's going on - if you can get an all-in-one kit that's awesome, if not, make sure you get kits to test pH, ammonia, nitrites, and nitrates. Those are the essentials. It's also a good idea to get a thermometer so you know what temperature your water is at; temperature affects how well the bacteria will grow, and also how dangerous high levels of ammonia are.
You don't need to get anything special to get bacteria into your system, either. The right sort of bacteria are everywhere, all you need to do is give them the right conditions to grow. If you test your water and tell us what you get, and let us know what your system is like (Charlie's right, photos are awesome), we can help.
For now, stop feeding your goldfish - if your 'levels are high' that's one way to get them down, the fish will be fine. Also, how big are the water changes you're doing and what water are you using?
