What is the analog pin for on a Phidget 3051 Dual relay board?
From the documentation, it looks like there is a buffer on the relay board.
From some googling around - On the phidgets forum:

It only needs 5v and GND from the analog. You can just wire up a standard sensor cable to any 5v power source, and plug it into the relay board.

Also:

The black connector is for connecting the relays to +5V and ground. It just simply provides an easy way to get power from the analog inputs.
The terminal blocks are for controlling the relays and labelled 0 and 1.
I agree that the Product Manual should be more clear.

It's worth noting that the documentation for that board is terrible. I can't find the schematic anywhere. I'm guessing as to how the board works, based on the fact that it seems to have two ICs on it, and the controller it's connected to doesn't seem to have the facilities to drive the relay coil.
Really, I would strongly suggest you e-mail the manufacturer, and ask for the schematic. If they refuse, return the board, and don't purchase from them again.

Re: &quot;Load Noise&quot;
This is incorrectly titled. What they are really talking about are inductive loads.
If your fish-tank lights use electronic ballasts, you're fine. Also, your bulbs are small enough that even with magnetic ballasts, I think you can probably get away without any sort of inductive kickback suppression.
You can tell if you have an inductive or electronic ballast simply by weight. If the ballast feels like it's got a bunch of steel in it (the ballast inductor), it's magnetic. If not, it's electronic.