I've got a few questions that I could really use some help with as someone who is new to all of this. If you have some time to help, I'd be incredibly grateful. I'm spacing it out under different headings for ease.
New Tank
I've purchased a Aqua 30 tank that's 24 Litres. I'm being told many different things about how many fish I can safely hold in this size tank, and it seems from most posts on the Internet people either have a lot smaller (bowls) or a lot bigger tank sizes than mine, so I can't seem to figure a rough amount out. I'd like some Neon Tetras (I do have a heater, already heating up the tank as I type) and maybe some Zebra Danios, as I've heard both are hardy fish. Will these two species live together and how many is recommended? I don't want them to suffer due to Ammonia poisoning or over-population.
Nitrogen Cycle
Someone explain this to me in basic terms. I'm mostly confused because most descriptions of it are using American measurements and I'm in the UK, so it's puzzling me further. I was told by the shop that a week is enough to power a nitrogen cycle through my size tank and to add food to it to help it on its way. Is this correct information? As it's described on here, treatment things are needed to speed it up - because my tank is quite small, how long would it take to reach a sufficient point in its fishless cycle on its own?
If that is not the case, can someone break it down for me into something that I can do step by step with this page open? Most explanations seem to diverge onto different topics mid-flow or give me products I can't buy in the UK...
Filter Problems
This might just be me, being someone new to all this, but I'm not sure if my filter is functioning correctly. I can't identify the brand of filter specifically, as I have purchased it and the tank from a friend, but I know that the filter came with Mars Fishcare Europe Rena Crystal Cartridges Size 1, if that means something to anyone?? I've put the Cartridge in the top compartment, the black mesh material in the bottom compartment and set it off. Only thing is it doesn't seem to move a lot. I can see water cycling, but not at any great speed. Is it supposed to be quite slow? I imagined if it needed to clean out Ammonia etc it would need to have some speed to it! It's fully submerged roughly 2 CM below the water line currently and there's very little to see for flow coming out...is this normal?
I know that, being new to this, it's trial and error but I don't want to end up with what is described as New Tank Syndrome in my first few weeks. Any advice I would be grateful for, I feel like I'm banging my head against a wall lol.
Thank You for reading and feel free to remove this post if it isn't allowed...
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.
(My air pumps are all from Rena), but I have no experience with their filter. 
