It doesn't slosh the water over the edge like all other magnetic scrubbers do, but it is not safe to use with plastic tanks as advertised. There could easily be another type of scrubbing surface used as an option for plastic tanks, but I haven't found any. I actually tried it on the plastic tank with a weaker magnet on the outside, but it still got scratched up. Still, this is the best algae cleaner that I have used in my 20 years of aquarium experience