Hi Natalie, welcome to the forum!
Sorry for the slow reply. I guess everyone was busy to have fun during the days off.
To answer your questions,
1. Your fish tank
24L is about 6.34 US gallon. It is just above the 5 gallon minimal recommendation. Anything smaller than a 5 gallon tank will be hard to keep the water conditions stable. So it is adequate as long as you do not overstock your aquarium with too many fish or too big fish.
You can have maybe 10~12 Neon Tetra in your fish tank at most. While it is also depended on your filter system and frequency of aquarium maintenance. Obviously better filter system of higher GPH will allow you to keep more fish in the tank of same size. Same goes for if you do more partial water change more frequently as well as feed less fish food to avoid pollute the system.
But of course, you need to cycle your aquarium before add any fish at all.
As for Zebra Danio, they are really active and fast moving fish. They would literally zipping around the tank. So a fish tank of 24L (6.34 gallon) might be too small for them if you want to see their natural behavior.
2. Nitrogen cycle
The size of your fish tank has nothing to do with the time required to cycle it. One week is not enough, especially when you are using fish food method which takes time for it to produce ammonia to start the cycle in the first place. It is important to understand that to start a fishless cycling, you need a source of ammonia. Thus the fastest way to do a fishless cycling is just to use a pure ammonia product. Even if you use pure ammonia, you can't complete a fishless cycling for at least 4~6 weeks (unless) you use a live bacteria product such as Tetra SafeStart to jump start the cycle.
Another important thing is, you need a water test kit to measure all the readings such as ammonia, nitrite, nitrate, and PH.
3. Filter,
I can't be sure what type of filter you are using. If the cartridge is made from the same company, then it is Rena. Rena is a good company as far as my air pumps are telling me.
(My air pumps are all from Rena), but I have no experience with their filter.
As long as the filter intake is under the water, and you see water flow, then it is working.
Feel free to ask any question. I will do my best to answer them.
