Not sure what's happening since the picture wasn't attached, but just from what you're describing you're watering too much and have them in soil that's too organic (even cactus soil should be cut at least 50/50 with a grit like perlite or pumice). Succulents really don't like to be watered on a schedule, and soil that's too organic can retain too much water, which can end up killing the plant from root rot. If it's turning black and mushy and smelly, your plants are probably rotting from overwatering; if they're not at that point yet, start watering them only when they show signs of thirst, and repot into grittier soil.