HELP !
My winter pond de-icer leaked!!
killing ALL my beloved goldfish

How can I test/treat the water for new fish ? ?

Hi Tomtree! Welcome to the board! What do you mean that it leaked?? What leaked out of it to kill the fish? What kind of deicer is it? I am sorry to hear that :(

I'm sorry about your godlfish.
I've not heard of a de-icer containing something that can leak. What leaked? For most things like oil, the best thing to do is to
change all of the water if possible. Use paper towels to collect things like oil from the surface and liners. Put fresh activated
carbon in a mesh bag in the filter to help remove other organics.

Ok, you gotta explain this one. My first de-icer had a broken seal and literally sunk to the bottom. We tried to dry out the inside styrofoam but we threw it out instead. Do you have a ground fault circuit to your pond? Did it actually leak something out, or did water leak in and end up electrocuting the fish? In our case it tripped the ground fault so we had no deaths. Did you take the de-icer apart to see what actually happened to it ( since it's trash anyway ). The basic concept for a de-icer is a 'stove heating element' and something to make it float.
Go to your local fish store for a quick water test supplies, pond store is better.

Pondmaster floating winter pond de-icer

O. K.
Now I've got to investigate further.........

All I saw was a brownish, discolored circle around the de-icer in the ice, which apparently still worked. Upon removal it has black, 'graphite-like' stains on it,
And a slight "oil slick" when the ice thawed.
The water cleared after slowly flooding out the pond, but now there is a white chaulky
substance toward the bottom when the water is disturbed.

Basically my question is should I bother filtering / testing the water or just drain / clean it and start over again?

In my area since I have to pay for water AND sewer together, I would test the water first. I had a pump blow up on me underwater...now that was an oil slick. Surprisingly enough it didn't kill any of my fish, just floated up and over to the rocks. Even if you drain and clean, testing the water first before you do that might give you 'a heads' up if you have to keep an eye on something ( ex. the ph ).

Well, I found photos of the de-icer in question but, wouldn't you know, none of the sites say what is actually inside it. How big is the pond? How many fish were there? Did other animals die too? I wonder if the goldfish could have died from something else. If it was from the de-icer, I would ask PondMaster (if you can find a way to do that). It's a pretty bad de-icer that would deadly. My de-icers get salt buildup and corrosion all over them. Some of that white salt and corroded metal flake off into the pond. Could that be the stuff you see on the bottom? It wouldn't hurt to test the water for the basics (pH, ammonia, nitrite, hardness) to at least know if they were affected or part of the problem. It's likely that if the de-icer actually leaked something, you wouldn't be testing for that. If the pond is small enough to do so, I would fully clean it out to be safe.