If it is glass use a fresh razor blade... you can use mineral spirits or paint thinner, but you can't get it on the silicone... my advice would be to use paint thinner or acetone in the center of the tank to get a spot started, being careful not to let it run on the silicone... then liberally spray vinegar on anything left around the dge or silicone... let sit 5 mins, then liberally re apply vinegar and use razor blade to clean anything that may not wipe off.
Don't windex or similar clean it... Acetone and or vinegar only... if you use acetone do not get it on the silicone.
the thing to not scratching is keeping the glass wet... and dont dig or gouge at the paint with it, lay the blade on a 20deg angle (roughly)