Do you have a heater along with the filter? Goldfish don’t need a heater for a tank, but tropical fish do.
Be very careful of simply relying on store help. Some employees are very knowledgeable and helpful. Others are dumb as dirt, or willingly provide completely wrong info. Still others will highly recommend whatever their stor happens to sell.
The need to condition/dechlor your water depends entirely on where you get your water from. I just finished my weekly 30% water change straight out of the tap, with no dechlor. The several year-old fish in all of my tanks had no problems. Gotta love Chicago water.
They gave out fish at your son’s prom? 
They had little fishbowls with betas in them at the tables. My son brought his home.
I’m going to get some library books on fresh-water aquariums and I sent Ivylad a couple of links. He was like me when I told him about the nitrogen cycle, “huh?”
So, we have some book-larnin’ to do!
I bought two little “Comet” goldfish and a basic 10 gallon aquarium two years ago for Christmas. I thought it would be neat for my kids to learn about keeping fish. Plus, I knew that cheap WalMart goldfish basically had a life span best measured in weeks. This way we could slowly work our way up to cooler, neater fish. Maybe even some sweet tropical fishes.
This summer I had to go buy a 20 gallon aquarium because those 30 cent each goldfish out grew it. I just checked, they are each about 6 inches long. My cat Sally used to sit there for hours plotter her next meal. Now she runs off when one of them jumps out of the water and hits the hood door. If they get too much bigger I won’t be able to flush them when they do eventually die. How do you do a “burial at sea” for a deceased fish that is to big to flush?
When this angelfish finally kicks the bucket we’ll actually have to do a land burial, I guess. With a hole and all. He’d clog the toilet. He’s alone because he ate his tankmates.