PH, Ammonia etc.

Question
Hi,
I have owned goldfish before, but I want to try something different, so I'm going to get some tropical fish. I have an idea of what I want - 1 betta fish, a few Cory Cats.
I was wondering if these are compatible, and if so, how many Cories I should get?
I am getting an AquaArt 60L tropical tank. I am talking UK size (not sure if American is different). Do you think this will be big enough?
Also, people talk about the PH, Ammonia, Nitrate and Nitrite levels - but I don't understand.
I know that I have to dechlorinate the water, but how much dechlorinator do I put per Litre?
Also, what are the ideal PH, ammonia etc. levels for Cory Cats and Bettas? How do I check what they are?
People have also been saying I need to let the tank 'cycle'. Does that mean I have to leave it running for a while before adding any fish?
I'm not sure whether I can get Betta fish (or cories) yet (there were a few cories in the pet store last week), but just incase I can't get hold of the cories, what should I put with the betta?
Likewise, if I can't get the betta, what should I put in with the cories that would be suitable for the tank size?

Thanks for reading - even if I have a lot of questions! I just don't have much experience with tropical fish (plus I'm only a kid).

Many Thanks,
Lara and my future fishies!

Answer
Well, first we should discover the liter to gallon difference. In the U.S. we go by gallons. I put 60Liters into 15 gallons which is pleanty big enough for two corys and one betta. In fact its perfect! One very important thing you will need is an aquarium heater. Tropical fish like warm water and you and I, not like goldfish who prefer cold water, yuck! Anyways, Bettas and Cory are compatible! Cory catfish are amoung my favs and I know a great deal about them, perfect fish for the first choice in tropical. I would question the betta though, as they can be picky fish to care for and sometimes fragile, ending up not doing well in tanks. Perhas a platy, guppy (super colorful), or a tetra would be better to begin. These fish are very hardy and give color and are super active, great beginner fish. I even have two guppies right now and I am a seasoned fish-keeper! Corys are also hardy and a great choice for beginners. In fact, corys clean the bottom gravel, they are bottom-feeders. Which is great for a beginner, they help but not eliminate some of the cleaning of the bottom of the tank. You could get three corys and they would fit fine in the 60Liter tank! As for the pH, ammonia, and nitrate/nitrites, they are not super big problems for the fish you are going to begin with. The fish, except for the betta, are hardy and can usually live in a variety of conditions. I would not worry too much about these specs right now. However, you should set your tank up, including heater and de-clorinate, for two or three days. I realize you probably want to run out and get some fish, but trust me, you should let the tank sit for two or three days. How much de-clorinator? Well, it all depends on the kind you get. Get a basic de-clorinator which makes the water safe for fish and follow the directions on the bottle exactly! Avoid declorintors which have EXTRA so called beneficial bacteria, this will only add to the possible ammonia issue. Just get a basic de-clorinator good enough to make the water safe for the fish. And dont forget to add it, its super easy to forget! The only big issue you might face is ammonia. This is excess waste from the fish, including left over food, which gathers in the tank and builds creating nasty bacteria which is bad for fish. Its as if we ate poisoned food, the same effect is the case with too much ammonia in fish. However, this should be easily avoided if you do the proper water changes and keep the tank clean. Keeping the tank clean consists of: a 10% water change every two weeks (basically 2 gallons [3.78 liters] taken out and 2 gallons [3.78 liters] of fresh tap water which is declorinated added back to the tank, replacing the water taken out), replacing the filter media or cartridge every month, and cleaning the entire tank including syphoning the gravel and wipping down the walls for algae every 6-8 weeks. It sounds difficult but once you know what to do its a piece of cake. I hope I answered all the questions asked. Feel free to ask back anything I was not clear on or anything you forgot to ask. Its best if you know what to do rather than not be able to correctly care for the fish. I will always be here to help. I love corys and suspect you will too. I would not choose the betta though, look up the other fish I mentioned and see if you like those, if not let me know. Look forward to helping you on the road to an adorable freshwater tropical tank!