Ich does not survive without fish hosts. All you needed to do is leave the tank empty for 2-3 weeks, dose ammonia in the mean time to boost the filter bacteria, then stock the tank. You didn't have to change the media....
Now you'll be cycling the tank for 4-8 weeks, daily large water changes if you've got the guppies and want them to live and possibly ich too because it will be left on the filter/substrate and it will move onto the guppies.
Your initial stocking was not good.