The real reason I did it was to exploit the space. I'm planning a new bigger tank soon as I have run out of space and when the deal on the Pavona frag came up... well you know how it is . I could position these two well away from any neighbors, get them in the right light and flow, and not worry about compatibility with close neighbors. I bought epoxy covered magnets and attached to frags but check on ebay I've seen magnetic frag discs listed.