Tighten the top then try and unscrew the bottom. And next time when putting tank back together, tighten it as normal then give it a half turn backwards.
I love Nautili - I had problems early on with unscrewing. An alan key down the airhole and using that to turn it sorted me out. They can stick hard at times, very frustrating!
In all fairness i didnt over tighten it i learned that after the 1st time i done it the glass isnt flush with the metal deck so isnt tightened the whole way