I had two apparently perfectly healthy goldfish for about 4 years, living in a medium sized glass aquarium in water butt water on a North facing window sill. It may be co-incidence but they both died soon after I put in some water snails to make the tank look more interesting.
Now DD (just 3) is very keen to have new fish, but I have done some reading and realised I was doing everything wrong. Tanks should never be on the windowsill, you should never change all the water at once, there should be oxygenation, etc. (I did put in lots of pondweed.)
DH will only countenance plastic fish tanks, in case a glass one spontaneously shatters and impales DD , so plastic it will have to be this time.
I could put in on a table in a darkish corner of the room, but would need a light (obviously). I find that I should also have a pump and a filter.
Finances really are an issue - I can't spend a huge amount at the moment.
What is the mimimum equipment that would work for common cold water goldfish? I can get fish from a friend's outside pond, where they breed like mad.