Ok. Sounds like you're recommending I use 2 Oxydator "A"s. I think I'll start slow and use 2 catalyst in one and 1 catalyst in the other. I can increase when I get a new ORP probe. My last one fried when the tank overflowed last year. Thanks for all your help. I feel much more confident to get started now.The main difference is down to how long each oxydator lasts between refills. However with stronger solutions of peroxide the peroxide in the model A is more likely to be converted/ broken down due to its design. With the model D you only have the catalysts and the base to act with the peroxide. With the model A the peroxide has the whole ceramic pot which also acts as a catalyst for any peroxide that escapes into it making it a safer and due to the amount it holds longer lasting between refills.