There's a lot of questions we are going to have to try and help you work out what's ging on, and if you're new to fishkeeping this may seem a bit overwhelming. But it's an opportunity to learn a bit more about the complex needs of fish and how the tank system works, so hang in there
Firstly, if you really do mean 20L, you need to be upgrading (or taking the fish back) ASAP. For the fish you currently have, you'd be wanting to look for a 75L (20 gallon) or larger.
Did you do any form of cycling the tank between when you set it up and when the first fish were added? If so, can you explain how?
Definitely get your own water testing kit, IMO they're essential for any fish keeper and testing should be done daily when you have unwell fish. I would never trust a fish shop that just says the water is fine - what does fine mean to them?
What have you been doing as far as water changes and cleaning so far, and what products are you adding to the tank?
For now, assuming the tank likely isn't cycled and is well overstocked, I would personally be doing daily 50% water changes until you can get a bigger tank set up. Making sure to use a good water conditioner such as Seachem Prime or API Tap Water Conditioner/Aqua Essentials.
Can we get a picture of the whole tank? Also what sort of filter are you running and what is the temperature?
And one more confusing question - do you know the parameters of your tap water? pH and water hardness are important for fish with different types requiring different water.