I used a new blade in utility knife to score the long cut first multiple times with a straight edge and then cut the short ends last the same way. I find it best to clamp a piece of wood below the trim so you have something firm to press down on, just clamp on either end to the back glass of the tank.
Obviously lower the tank water level to make it easier.
You could also tape a plastic bag large enough over the water surface and up over the tank rim to catch plastic debris if using the multi purpose saw thing but not sure how the vibrations would be.
