Sorry guys but a typical dimmer won't work to solve your problem. Led fixtures operate differently than old fashioned incandescent light bulbs. Leds require a certain voltage threshold to be reached before current will begin to flow through them, where as incandescent bulbs will happily glow with the reduced voltage that the dimmer provides. Leds can only be dimmed by varying their current level not voltage.
You'll either have to replace you led lighting fixture with a fixture that already has a built in dimmer function-OR- it may be possible to "hack" the one you have to make dimming possible. Whether or not your fixture can be hacked depends on what type of constant current driver is being employed. It's possible that the electronics needed are already there, and the addition of a few strategically placed wires and a micro-controller will provide the dimming capabilities that you're looking for. I've hacked a few of the earlier led fixtures to do just that, but I'm not familiar with the "guts" in the fixtures that you're using now. You may also want to contact the manufacturer and see if they have any "off the shelf" options for you.
225 Gal, Glass "Reef Ready" w/ twin Iwaki Pumps & 2 Ocean Clear inline filters. Custom 320 watt LED Lighting system utilizing a 6' Maker's Heat Sink, 12 BridgeLux Vero 18's,3 purpose built 5 channel A6211 Led driver/Controllers, and a 20lb CO2 system w/ Milwaukee Ph Controller.