New To Fish!

I would not get a biorb as a first tank. Yes, they look pretty but they have their problems. You are much better getting a standard rectangular tank, especially if you want cold water fish.

Cold water fish means goldfish, preferable the fancy varieties, which means those with a double tail rather than the types with a single tail which really need to be in ponds.
For fancy goldfish you need 70 litres for the first one, then an additional 40 litres for each additional fish. They grow big!

There are fish sold in shops as cold water fish but they are actually temperate fish. Things like zebra danios and white cloud mountain minnows. They can be kept in heaterless tanks provided the room they are in never drops below 18^oC in the middle of the coldest winter night. If the room will get colder than that, you'd need a heater and if you had a heater you may as well look at tropical.
Zebra danios, despite being small, are extremely fast swimmers and need a tank that is a least 1 metre long. White clouds can get away with smaller, but nothing smaller than 60cm long.

Is there a reason why you want cold water fish? The only difference is that tropical freshwater fish need a heater. That's it, no other difference. And there is a much bigger range of fish, including some that are quite happy in smaller tanks. The real downside to cold or temperate fish for a newcomer is the size of tank they need.


Have you come across cycling yet? This is the thing that surprises newcomers most. In order to maintain a healthy tank and fish you need to grow 2 colonies of bacteria in the filter. These bacteria take care of the ammonia from the fish waste. There are two ways to cycle a tank, with fish and without. Without is much easier (no daily water changes for a couple of months, no risk of fish getting sick and dying) but it does mean no fish for several weeks and I can quite understand that with an 8 year old's pestering that might not be an option. You know your son better than me, would he be able to cope with not having fish if you were to "assist" him in testing the water and monitoring the cycle? "Assisting" of course means you doing it and him helping a bit 

So you can make up your mind, here are two threads (which you may have already found) which describe the methods for the two ways of cycling.
Without fish
With fish