Yes you can cut the tank down with a glass cutter but you have to take the tank apart before cutting it.
*NB* Wear a pair of safety glasses when doing anything with glass.
Use a single sided razorblade to separate the individual pieces of glass.
Clean the old silicon off the glass.
Put the sheet of glass on a hard flat smooth surface and use a permanent marker to draw a line where you want to cut the glass.
Use a metal ruler or piece of wood as a straight edge to guide the glass cutter along.
Use a glass cutter to score the glass along the line.
Remove the ruler/ piece of wood and glass cutter and put them somewhere safe.
Put a thin piece of wood under the glass so the edge of the wood lines up with the score/ scratch in the glass.
Have one hand on one side of the score. Put your other hand on the other side of the score and gently push on one side of the glass.
The glass should crack along the score.
Use a piece of fine sandpaper (wet and dry sandpaper is best) on a wooden sanding block, to gently go over the edge of the glass to remove the sharp edge.
When you have the pieces cut to size, silicon the tank back together.