I voted for 1 tank crash but then I read your actual post and notice your note about 50% loss of life as consider a crash.. Well if thats
the case then I've definitely had more than one!
Most of my crashes have been due to heat (ie. Stuck heaters). I'm hoping that I have solved this by putting my heater on a controller. Controller will give it power if it decides heat is needed and it will cut power to it if no heat is required. In theory this should eliminate the stuck heater situation.