*Acrylic safe blade is necessarily soft and can therefore pick up sand or small bits of gravel if shoved too hard down into the substrate. User should check plastic blade surface periodically by running a finger or thumbnail across the surface. Some aquariums are made from low grade imported acrylic that is extremely soft and of very poor quality that can be easily scratched. User must exercise proper judgement in use and should test blade in an inconspicuous area prior to use to insure compatibility.