is right about one thing, they can be increadible at keeping the sanbed clean. By the sounds of it, you have a tiger tail cuke
The reason I asked about the tanks age was they rarely do well in new set up. Not only for the water stability issues but also natural food sources. If the tank is too young, there will not be enough to sustain it. Once in a mature tank (>1yr) they can do quite well and the tiger is propabley the best choice. If they do ever "meltdown", they are the least likely to poison the tank as long as the tank is not too small.