Old thread, I am sure by now last year's problem is solved, but it is probably going to happen again.
When ponds get really cold the bacteria become way less active, especially Nitrospira, the species that removes nitrite. This is what I was saying in my post of last year about Koi (goldfish, and other pond fish, too) showing Methemoglobinemia, aka Brown Blood Disease.
This often shows up in early spring. The fish are starting to become more active, but the nitrifying bacteria is not yet recovered.
By adding more of exactly the species the pond needs you might be able to help it recover faster. These are slow to reproduce, especially in the cold weather.
Prime is available as a powder, called Safe.
Go to the web site and see if it also neutralizes nitrite and nitrate. I think Prime was not originally labeled for that, but they discovered it did neutralize these. Prime costs a lot less than Amquel or Amquel Plus, and Safe is cheaper than Prime. Also look into the Amquel product line (Made by Kordon) for ponds. Maybe more economical in the pond size packaging.