I use two of the HD models. You don't need the controller interface. I would order some extra fuses when you buy one. The HD model is barely capable of the load generated by 2 400 watt ballasts (though its rated to 15 amps). The fuse Neptune chose to use is not a standard size and pretty much impossible to find anywhere (found some of lower amperage but the right size at Fry's Electronics). I can't understand why they wouldn't have chosen to use an automobile size fuse that you can find anywhere.
Also, I would point out that despite having the Aquacontroller III and the 2 Direct Connect HD (?$1200 invested), my controller still turns some of the lights (especially the VHOs) off/on randomly at times. I have completely reprogrammed the thing to make sure that there are no duplicates (as recommended by Neptune) and it still happens. Very frustrating.