There’s no decent reason. 100 years ago some asshole wrote that formalin will settle to the bottom in ice cold water.
Which is actually true, it will stratify if you leave it in a vial under test circumstances. But the concept is that it would create hot spots on the bottom of the pond, except that there’s usually at least some circulation to keep that *theoretical* event from happening. I wouldn’t spend a moment worrying about it.
If you are applying the formalin into water, that is below 40, and it clouds as soon as it hits the water, it’s not going to work, because it is forming paraformaldehyde, which, in the big scheme of things is no big deal, except that the cold water has deactivated that particular formalin recipe.
Also, paraformaldehyde is toxic to fish. Twice as toxic as regular formalin but it’s not going to kill a whole Pond.
Most commercial preparations the formalin has been buffered to prevent that.
For what it’s worth, because of high oxygen tensions not available in summer: I love formalin in cool water.