I'm sorry, I completely understand when you're new to the hobby that it makes sense to trust the advice of fish stores, especially a big chain. Iogically, they should know what they're talking about, and give sound advice, so it's not your fault you believed them. But the hard lesson a lot of people only learn when their fish start dying is that fish store advice is terrible advice, especially from the big chains.
Fish keeping is too complex to break down into a points system. That's a pets at home thing, they try that to create some kind of basic standard across stores, but it doesn't work, and at the end of the day, animal welfare laws are rarely applied to fish, and the store is there to make money, even if it means selling you a tiny tank and fish that can't live in it. Many of the employees will have never kept a tank, wouldn't have the time to explain what the nitrogen cycle is if they even know, and need to sell you fish and fish accessories. They place some limits, but not nearly enough since it wouldn't be good for business.
But do some research on mollies. Look at how large they get. I don't know which species of molly you have, but your average black molly female can reach 12 cms in length. Measure out 12 cms, then measure how wide your tank is. Your tank is less than five gallons of water, and has very little horizontal swimming space. So size alone, bearing in mind that mollies keep growing and yours might be old enough to reproduce, but likely aren't fully grown maximum size yet since you just bought them... they're going to outgrow your tank if they haven't already.
I'll give you an example from my father's tank for scale. See that yellow fish on the left? and the black fish right abve it, about the same size? That's an adult yellow molly, and an adult black molly. Probably about the same size as yours are now, roughly. The much larger black and silver dappled fish below the yellow one? That's a fully grown adult black molly. They can get BIG. This tank is 57 gallons, yours is less than five. That fully grown adult molly would be so cramped in there, and you need to consider the whole lifespan of the fish, and how active they are when they swim. Mollies are strong swimmers, and active fish, on the move all day. They need space for that.
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Seriously Fish is a trusted resource, written by scientists about fish care requirements for different species, have a look at the information about mollies here, especially the minimum tank size.
www.seriouslyfish.com
The other aspect of a small tank is that it's very hard to maintain suitable water conditions. Did the store explain tank cycling to you? I'll link a video, but in a nutshell, fish eat/breathe etc, they excrete ammonia. Leftover food or rotting plant material turns into ammonia. Ammonia is very toxic to fish. In the wild this isn't a problem since the waste is diluted in huge volumes of water and bacteria break it down, but in a small enclosed ecosystem like a tank, it can become a big problem, very fast. Ammonia burns fish, it burns their gills, their skin, damages them internally if they're forced to swim in it. We have filters mainly for this reason, to grow the kind of bacteria that converts ammonia into less toxic compounds, making the water safe for the fish. Cycling a tank properly takes weeks, usually longer than a month, but pets at home tells people to set up a tank, wait a week, then add fish. This does nothing to actually cycle the tank, and leads to a lot of fish deaths. I'm hoping that since you're here, it won't happen to you, now you have a chance t learn how to cycle the tank and make sure it's safe. Do you have a water testing kit?
That's a really simplified explanation but it's late and this reply is already super long, I'm sorry! This video gives a really clear explanation of what the nitrogen cycle is, and why it's so important in fish keeping.
So your two mollies will be producing ammonia. In a 15 litre tank, even a small amount of ammonia is very risky, since there's so little water to dilute it. It would take two fish a long time to produce enough ammonia to make the levels toxic in a 200 litre tank, but a 15 litre tank is going to reach toxic levels much more rapidly, as an example. So you'd need to do very regular water changes for the water to be safe for them, even once the tank is cycled.
Sorry, that's a lot of info to dump on you, and I don't know how much of it you might already know. For now, you can help the fish by doing large daily water changes. It's likely that the fish is clamping its fins and sitting on the bottom because ammonia and/or nitrites are too high, and it's stressed and feeling the toxic effects. If you change 75% of the water, making sure to use a water conditioner and temperature match the new water to the tank water before adding it in to the tank, it should help the fish get by until tomorrow. Do those daily for now, while we see where you're at in the cycle.
In the long term, a larger tank, or different fish will be needed. But for now, large daily water changes are the best thing you can do for them. Also, be careful not to overfeed them. Too much food means more ammonia.
Sorry for the essay, will try to check in again tomorrow!
Fortunately the bacteria live on every hard surface in the tank, in the substrate, tank walls, on decor, so hopefully there will be enough bacteria to help re-build the colony quickly. Some of the bacteria deep in the filter might have survived too, so chances are you're not starting over from the beginning, but the colony will need a week or so to regrow enough to handle the bioload of the tank. When something like this happens, we call it a mini-cycle, since you're unlikely to have wiped out all of the bacteria, but they still need time to rebuild to a good size.
