The unit comes with an L-shaped piece of plastic that works as a lip that fits over the top of your aquarium. It fastens to the Reactor 1000 so that the bracket faces the aquarium, the lip fastens to the bracket so that it sits over the glass edge and the reacter sits behind the bracket on the side of the bracket that is opposite the tank side. This is the easiest way to mount the reactor on tanks with glass under 3/8".
Or you can leave that liip piece off and hang it on a nail or screw set into your stand.
For aquaira with glass thicker than about 3/8", you'll need a different "lip" piece that doesn't come with the unit but can be gotten from AB through any of it's vendors. It allows a longer reach for the lip over thicker glass. This Reactor 1000 bracket is essentially the same as on the phosphate reactor and the bracket that AB sells for mounting items in a coral tank. Some of those other items come with the heavier "Lip" piece on the bracket and some do not.
Truth is, so long as the REactor 1000 is securely clamped to the tubing at the fittings and is near vertical with respect to gravity, it doesn't much matter how it's attached, you could even strap it with velcro or just let it hang -- it will work just as well.
Just keep it out of sight but where you can get at it if you need to -- sooner or later you'll need to clean the tubing

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Good Luck,
Scott H