Posted: 10/02/2013 at 13:07
My cacti are over 20 years old and have always been watered with tap water - I don't think that was your problem, Brian.
Baby cactus - make sure it's in very free-draining soil (water should run through almost instantly) in as light a position as you have. At the moment, make sure it's not too cold - 15C or so will be fine. With regard to watering, give it about a tablespoon of water every few days on top of the compost near it - try not to get the base of the cactus wet unless it's warm enough to dry quickly.
They are actually quite easy to care for in summer, but you've picked a more difficult time to get it started. Good luck!