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What should you feed your koi? How many times per day? Is Corn really that bad in a Koi diet? What are the most common feeding mistakes people make? What's the best food?

Freezing alter the chemical properties of Formalin
Formalin that has been chilled to freezinf or near-freezing may form Para-formaldehyde.

Formalin Chilling Freezing Effects
Chilling Formalin - by Erik Johnson DVM
"Does freezing alter the chemical properties of Formalin when frozen, which would make it lethal?"
But, suffice it to say that when you freeze or even bring Formalin NEAR freezing it's chances of morphing into Para-Formaldehyde are about 99% but it doesn't always happen. Para-Formaldehyde is highly toxic to fish. I have an unfortunate experience with that. Actually several because I could not see through the bottle, and thus did not see the PF precipitate.
How could you tell? Well, if the Formalin has OPACITY or "white cottony stuff" in it - then it HAS made the change and IS toxic (lethally) to fish.
If you freeze Formalin or it gets near that temperature, and it does NOT go whitish or opaque, then it's still safe to use.
In other words, you can definitely tell by looking at it. "Bad" or chilled formalin will have whitish precipitate in it - and SHOULD NOT be used.
Good question

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