It sounds like the manager was correct....in part....Your tank doesn't sound as if it has cycled....Have you read the articles here about cycling your tank and the nitrogen process?
http://www.fishlore.com/NitrogenCycle.htm This article explains it all. What you don't want to do is add more fish until the cycle has finished.
At this point what I would do is go to the local fish store and get some "Prime" water conditioner and do a massive - 50% water change using Prime as the water treatment. Prime will lock up toxic ammonia and nitrite in the water making it easier for the fish to tollerate. If you can't find Prime try to find Amquell+. You will still have to perform frequent water changes of 25-30% to keep the water chemistry in check during the cycle. Just don't vaccuum the gravel or perform filter maintenance during that time.
You should also try to get (when you can or if you haven't already) a good test kit. Many of us here use and recommend the Aquarium Pharmaceuticals Freshwater Master Test Kit. (petsmart.com usually has them for under $20 and I've heard that the stores will honor the on line price with a print out) These kits are far more accurate than the test strips that many stores sell or the ones that come with aquarium kits.
If you don't have a kit, take a water sample with you to the store and ask them to check: pH, Ammonia, Nitrites and Nitrates. Write down the results and post them here so we can help further. Good Luck!