Hi there and Welcome to TFF!
Beginners who stumble across our forum are often already in distress over what's happening to their new little pets and then our posts come as quite a shock sometimes as they begin to realize that the hobby is quite a bit different from what they thought and that neither the equipment instructions nor the pet shop really said anything at all about expecting these kinds of problems.
It *is* a little daunting at first, but the members here are great and fun to talk to and the information you start learning is itself quite fascinating if you let yourself get interested in it. The whole hobby is often very different from anything people have previously experienced, like a normal pet or a fast-moving technical hobby, and that's also part of the pleasure.
As the members have pointed out, there are a couple of problems right off: you bought a tank with a built-in filter and have experienced right away that lots of equipment in the hobby is not completely "thought through." In your case the filter setup can trap fish, which is never good. The other thing the members are talking about, the "cycle" is the thing they know is looming as your biggest problem.
When you buy a new filter in a pet shop it comes with instructions and you put it all together, get it running and feel like you have finished the correct "Setup" as far as you are concerned. This is a wrong impression. Its really a raw kit that needs the attention of someone with some fairly arcane knowledge about how filters really work and it needs anywhere from 1 to 3 months of attention and work performed by the person with this knowledge.
A filter performs 3 main functions: mechanical filtration, chemical filtration and biological filtration. Mechanical filtration is the easiest to understand. We all expect the sponges and floss and things we put in our filter to catch all the little debris particles in the water. The chemical function is special. Its an optional function that we can use if we need to remove medications or remove yellow tannins caused by driftwood or unusual organic smells. Chemical filtration (charcoal and some other things) is not needed on a normal basis.
The third function, that of biological filtration, is the surprise one for beginners and is by far the most important function. The media (sponges, ceramic pebbles, ceramic rings, "bioballs", polyfloss, etc.) is designed to have a huge surface area and lots of places to trap things. The big surface area serves as a special home for beneficial bacteria that needs to be grown in the filter. This business of growing bacteria is what takes the long time I talked about by the way. In fact we actually try to grow two very specific species of bacteria, for freshwater tanks, and the bacteria form sticky "biofilms" on all the media surfaces (not to mention the rest of the tank!) These biofilms enhance the debris catching of the mechanical function of the filter. But its the bacteria themselves that are really important. They carry out the stages of the "nitrogen cycle" that the members talked about above. Several of our pinned articles explain the nitrogen cycle and I hope you'll find it as fascinating as I did. Its really a little world unto itself and the more you understand it, the better an aquarist you'll be in the long run.
Anyway, good luck with your studies in this new "biology class" you've stumbled upon. I hope you'll ask questions about the nitrogen cycle and about the test kits you'll need as a first thing. Test kits would really be the very first thing you'd be studying when you walked into the store to set up a new system if you "only knew!"
~~waterdrop~~
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