Personally I would drain the tank and refill it with all new good to go water. Partial changes you're just diluting what is already in the tank...not removing the problem 100%.
Drain it, take out all of the rock you don't want in there, add in the new rock (after rinsing it off really well) and start the refilling process. Once that's done, get the temperature where it needs to be, add some cheap livestock and your live bacteria and see how it goes for a month.
Drain it, take out all of the rock you don't want in there, add in the new rock (after rinsing it off really well) and start the refilling process. Once that's done, get the temperature where it needs to be, add some cheap livestock and your live bacteria and see how it goes for a month.