whats the best way to get rid of hard water marks on glass?

As long as you are only working on the outside of the tank, try vinegar or vinegar and water on the stains. Just don't get the vinegar in the fish tank.

There is no simple way to do it (like washing) it will take elbow grease. I've found Mother's Carnuba (WITH cleaner) applied by hand, then removed with a clean, good towel. The harder you work, the better the result. Keep up with it using strait Mother's Carnuba - my favorite wax ever, and I have used them all. This keeps water spots from forming again. Never, ever use an electrical buffer. Ever. Never. Unless you prefer ugly swirls over water spots

Put a little toilet bowl cleaner on a Q-tip or a small rag. Rinse very well. And use only on the outside! I've used this cleaner on shower doors too.

whit vinegar and very hot water