I usually shock my in ground 18000 gal pool with a couple pounds of shock, but having some trouble getting it cleared up this year, so I decided to use 3 pounds. I had some shock from last year and a new container I just bought so, to use up the old, I put in 2 scoops of it and one scoop of the new one in a large bucket (I regularly have done this in the past with a single brand of shock, no problems). Added water by scooping in the pool, immediately the contents started boiling up bright yellow and orange, threatening to overflow the bucket. I was not sure why this was happening so didn't look like something I wanted in my pool and I quickly set it out in the yard. I appeared to be producing large quantities of chlorine gas which spread out onto the ground and killed a circle 20 feet across in the yard. Looking at the shock containers I had 2 scoops of sodium dichloroisocyanurate dihydrate (99%) and one of Calcium hypochlorite (78%). Got a whiff of this in the process and can say it was life threatening to breath (no joke). So, is mixing these a known bad thing to do? I looked around the net and see sights that say no problem using both in the same pool, but no mention of the mixing part.