Sorry I can't remember what it was called but when we had ours done they were insistent that we needed the chemical added - I'm still a bit dubious but time will tell I suppose - no problems yet but only been down a couple of years x
Thats what I did for mine too, been down since the late 70's for 2 boxes & the early 80's for the other 3 stables.
No problems, tho havent had any equines in 2 of the boxes for 5+yrs, but the other 3 are still fine & dandy