Sorry MissBec, but based on your post Ammonia won't go down? Need advice?
it doesn't seem like you were done fishless cycling. You were getting there but not done yet. Fishless cycling is done when your aquarium can convert 4 ppm of ammonia to nitrates in 24 hours.
First thing, absolutely no more fish. 20 liters is about 5 gallons. That's enough for a Betta and a snail, but not anything else. I know you said you're planning a bigger tank, but we have to deal with things right now first. That's just too much bioload.
lilnaugrim gave good advice with keeping up the water changes. Make sure that when you do you are using a dechlorinator that will lock the ammonia. This will be safer for the fish, but will give you a false high reading. That's ok. Fishless cycling takes work, but it's not the end of the world and people on here can help you through it.
One other thing, not sure if you have plants, but they will help. Floating plants especially since they will grow quickly even with stock lighting since they are so close to it.