Does O+ play a roll in fish death? Meaning if too much is placed in the substrate will it poison the water per say? I have been using O+ now and when I first used it is when I had all the deaths in my 75 gallon. My 29 gallon had similar deaths as well. Just a thought. Now that time has passed and WCs have been made I haven't had any problems except with Otos. From what I hear they are a pain in the butt to keep alive anyway.