I also use a pair of plant scissors. I got them from DrsFosterSmith and use them for only the aquarium. I also use fingernails on thin stems. My tanks are small, so it's easy to take the stem plant out of the tank, trim, and put back in. The scissors themselves rarely go into the tank, but with the plant scissors, I don't think it matters. With other scissors, just check to see that they aren't greasy at the hinge.