Since I had the fish indoors last year, I am unsure as to when to put the de-icer in the pond? The water is still totally open and it is moving really well with the aerator. Do I wait until it starts to freeze until I put the deicer in? That's what I am planning to do but thought I had better ask first :P

Your pond tent and aerator must be working pretty well! What are the average low and high temperatures there now? They're below freezing most of the time, right? Even if the pond is still liquid, put the de-icer in before it freezes. It will help keep it more open longer. If you didn't have the aerator, and it froze over, it would be pretty hard to put in the de-icer. If you are worried about saving money, then put the de-icer in but don't turn it on yet. The de-icer should only come on when it nears freezing so it shouldn't waste electricity until then. I live in Zone 6/7 and will be putting in de-icers sometime in early December.

I just put my de icer in last weekend, my de icer kicks on when the water temp drops below 40*. I live in zone 7 at the NJ shore. I also have a little bubbler going this year too. I have a shallow pond (2 feet) and I want to make sure the fish live. I did not have fish last winter.
Our weather last year and this year so far has been crazy. Some nights it drops to 32* and some nights it is 55*. Last week it was 45* in the day, yesterday it was 67*.
So I figured better safe than sorry.

KK which de-icer do you have?

I have the Thermo-Pond models, I usually don't put my out until I see ice. Even frozen over (not too thick) you can set them on top the ice; they'll melt through.

I have the thermo pond one too. That sounds good, thanks for your advise!

You can put the deicer in anytime. They have thermostats in them so they will only come on at a certain temp and then shut off once it reaches the temp again