I use one of those box-cutter blades. I just hold the bare blade in my fingers. I like the box-cutter blades because the top edge isn't sharp so you can hold it, and the bottom blade is nice and wide. It's made to be "flipped" (change the cutting side) in the box-cutter handle, so the blade itself is a nice size. Don't press too hard at first - press gently and see how much pressure you need to apply. I don't push very hard at all for my tanks. The green spot algae is the toughest to get off, but even that will come off pretty easily with minimal pressure. I don't get too close to the silicon corners, though. Since I'm looking at an angle through the glass and water, I can never tell how really close I am to the silicon, so I leave it alone with the blade. I put some Excel on a paper towel and rub the corners with the paper towel.