I am no fish expert, but my son had a cold water tank in his room for some years, which I ended up looking after.
We fed them once a day, and I used to clean the glass with one of those magnetic things and the filter weekly using some water out of the tank so as not to lose any of the goodness in the water, and replace a third of the water every two weeks or so. I also heard that you should leave the tap water to stand for 24 hours or so before you put in the tank, and used a water purifier liquid. We still have some of the fish in our pond now, 10 years later and going strong.