Hi Fuzzydonna
I'm afraid I have some bad news for you.
10 litres is a very small tank and, despite what the manufacturers say, it is not suitable for any fish. Mollies in particular are big fish - females can grow to 6 inches long.
But for now you need to keep the remaining fish alive until you decide what to do.
When you say the nitrogen level is high, do you mean nitrate? And what are the levels of ammonia and nitrite?
The way to get any of the three down is by water changes, and I would do more than 20% every other day - I would do at least 50% every day. And if you have readings of ammonia and/or nitrite over 0.25 ppm, you need to do enough water changes to get them below that figure and keep them there. Maybe even multiple daily water changes. As long as the new water is dechlorinated and warmed to approximately the same temperature as the tank, the fish shouldn't suffer.
And I'm afraid to say that I do not know of anything you can add to the tank that will lower nitrate except perhaps a lot of live plants.
In the longer term, I would give serious thought to the tank. You really have just two choices. Either get a bigger tank - and for mollies this needs to be at least 30 inches (75 cm) long - or rehome the fish and get shrimps. 10 litres is too small for even a single betta (siamese fighting fish)
I'm sorry to be the bearer of bad news.
One last point - don't believe anything a shop tells you until you have researched it for yourself. Some (particularly large chain pets stores that sell fish) don't train their staff in anything but selling techniques. Others will say anything to make a sale. The health of the fish is low in their priorities.