First off, I would avoid that fish store or at least ignore anything they tell you. Sounds like they set you up to fail. They may be nice people, but they should not be used as a source of advice IMO.
Fish create waste. It manifests as poop as well as ammonia via their gills. The poop will decompose and release more ammonia. This is not a problem in a natural system like a lake or river, but in a closed system like an aquarium we have to monitor the accumulation of the waste and remove it before it reaches toxic levels. In a "cycled" aquarium there are bacteria that consume the ammonia in the water, which protects the fish from it's toxic effects. However, after these bacteria metabolize the ammonia they leave nitrite behind, which is also very toxic. Other bacteria then consume the nitrite with the end product being nitrate, which is much, much less toxic. There are no bacteria in our aquariums to consume the nitrate, so we must remove it ourselves. This is done with water changes.
Cycling a tank involves the establishment of the various bacteria mentioned. The bacteria in a bottle products are an attempt to instantly cycle an aquarium. Sometimes they work. Many times they do not. Some brands seem better than others. They are entirely unnecessary. The bacteria we want in our aquariums are in the environment and if you set up an aquarium they will populate it; "If you build it, they will come." They only need a source of ammonia to get started. If you cycle a tank using fish, they are the source of ammonia to feed the bacteria and get the cycle started. The problem with using fish to cycle is that they have to swim in their unprocessed waste, while the cycling gets established. This can be harmful to the fish. It's best left to experienced fish keepers that know how to keep the fish safe in that situation. With a fishless cycle, we provide the ammonia instead of the fish. There are various ways to do that, but because you are now doing a fish-in cycle I will forego the fishless cycle details.
When cycling a tank, especially with fish you need to be able to test the water for pH, ammonia, nitrite and nitrate. You should also know the KH (carbonate hardness) and GH (general hardness) so you know the character of your water so you can choose fish that are appropriate for the water you have. As was mentioned by others, fish can come from places with vastly different water parameters. For some fish, this matters a lot, while many other fish are quite adaptable to various water parameters.
When testing a cycled aquarium the ammonia and nitrite will be -0- because the bacteria are doing their job. There will, however, be nitrates; also because the bacteria are working, but because there are no bacteria to consume them, the nitrates accumulate. We use the level of nitrates as an indicator of when a water change is needed. Because they are not as toxic as their precursors, we don't need to keep them at -0-. There are different opinions on how high is ok. My opinion is that nitrates are zero in a natural body of water that's not polluted, so I strive to keep them low. A common amount that many try to stay below is 20ppm (part per million). I keep my tanks at 10ppm or less.
Weekly water changes is a good place to start. Many of us never stray from that frequency, but every aquarium will be different and once your tank is well establish you can decide to tweak that schedule if it's appropriate.
Water changes do more than remove nitrate, but nitrate concentration is the indicator we use. Water changes also remove other things as well as add back minerals that have been depleted.
So I suggest you start by doing a 50% water change ASAP. Use water that you have dechlorinated and it should be at or close to the same temperature of the tank water. A couple degress F either way is ok.
Next, you need a way to test the water for ammonia, nitrite and pH at a minimum.
Test strips are not great and often don't test for ammonia. They are quick and easy if used and stored correctly. Liquid test kits are best. The most common is probably the API Master test kit which tests for ammonia, nitrite, nitrate as well pH. When you're able to test you want to test for everything mentioned. Report your results here. A cycled tank will have zero ammonia and nitrite, but you may see some if your tank isn't fully cycled yet. That's ok, but keep them below 0.5ppm with water changes. So, for example, If you test 1.0ppm of ammomia, doing a 50% water change will reduce them to 0.5ppm, so a 75% water change would be better. You are doing this to keep your fish safe. If you were doing a fishless cycle you wouldn't need to do a water change at these levels.
The bacteria we are discussing adhere to solid surfaces, so changing water does not hurt them. The sponges and other media in the filter are a major location for them, so proper filter maintence is important. To clean the filter media, a rinse in the tank water your removing (water change) or in dechlorinated tap water. It's best not to clean filters or the tank when cycling. If you use a filter with disposable filter cartridges, evry time you throw out a used one you are throwing out a bunch of bacteria that you worked so hard to establish. Many of this style of filter can be retrofited to work better, but let's not worry about that now.
So do a water change now. Buy a test kit. Test and post results here. Also test your tap water for ammonia after it has been dechlorinated. pH , KH and GH of your tap water would be helpful to know as well. If you have to wait several days for a test kit you can do more water changes while waiting.
.
