I'm sorry you've been losing fish - that's horrible.
It is much more difficult to keep the water in a small tank pure as every single thing - e.g. over feeding or having too many fish, will make the water quality change quickly, and stress or even kill the fish. In larger tanks this is less of an issue simply because there is more water to absorb these changes.
22 liters is a very small tank. That limits the kind of fish you can keep. For example, mollies are usually recommended to be in a tank that is at least 90 litres. Even guppies will be happier in a 40 liter tank. The only fish you could really keep in 22 litres is a betta, and even they will do better in a bigger tank.
You say the tank is fully cycled, but you also say you have "next to no" nitrites. As long as there is measurable nitrites in the tank, it's not yet cycled. I know that's not what you want to hear! But this means you need to continue doing water changes to keep the fish safe from the nitrites, which can really harm them.
What kind of tests are you using? Be aware that strip tests are not nearly accurate enough. If you are using a liquid test kit, what are the exact nitrite and nitrate values you are getting?
What kind of filter are you using?
And be sure not to over feed, as that will affect the water quality.
You also don't specify what fish you have at the moment - more than 2 mollies and 2 guppies?