Get some thin pvc tubing. Cut a 45 degree slit into it using a scroll saw or similar thin blade. Cut the slit between 1/2-1" from the end at the most. Otherwise, you'll not get to the bottom of the tank before the pvc starts to poke into the substrate.
Insert a single edge razor blade into it.
I've made several of these. They work REALLY well! Except if you have a canopy, then you are limited to how long the pvc can be.
Are you far from Ypsilanti? If not, you can come over and I'll make you one to any length you want for really cheap. I even have lots of new razor blades.