Had a friend with almost same situation, we dumped everything, filter media, gravel, and then took it to his back yard, filled it with water (hose) and added like 2 cap full of chlorine. Stir around have a smoke.. waited an hour and dumped it all.
Next we rinsed it several times and let it dry. We also rinsed the decorations with some chlorinated water and rinsed with hose water.
After all that we bought new filter media and added a new bag of gravel and he was all set.. going good for a few months now.
Good luck!