The best place for most cactus houseplants is a warm, bright spot. These sun-lovers do best when they get lots of direct light. In general, you can water most types once every two or three weeks or so. If in doubt, it's usually best to water too little; cacti can rot if they stay too moist. Most cactus houseplants are slow growers and don't really need much fertilizer. Just a couple of times per year will do.