OR dup a bottle of viengar in the tank and put a couple gal
of water in there and start rubbing with a wet rag. DO NOT use anything abrasive obvuosly as you will end up scratching the glass. The viengar should disolve the stains fairly quick.
Rinse and Rinse.
I shy away from windex on the outside of a filled tank. The spay from the windex if not careful can go into the tank. Since windex usually contains ammonia then thats no good. Instead I just use a slightly damp rag and then buff with a soft towel.