I'd like to see how coarser bubbles would affect the size of the holes (coarser bubbles = bigger waves fro the bubbles hitting the surface). Also, in your pond, I would make sure the diffusers for the winter are set no deeper than 1/4 the total pond depth. Two reasons:
1) Leave a warmer water refuge for the fish (39°F-40°F)
2) That *should* possibly translate to warmer water being brought up to the surface than what you are seeing now, and that warmer water will melt ice faster, even 1°F warmer than what you are seeing now.

I'll take a picture of what my single diffuser "plume" looks like in the winter - it's at 3' below the surface in 10' of water right now. After doing a test with the Vertex diffuser disk, I swapped it out for a what I believe is a matala disk that I only run in the winter because it has a much coarser looking bubble plume. I've had this disk for quite a few years and don't remember who the mfg. is, all I know is that the winter diffuser is NOT from Vertex. I do have some diffuser disks here from MNFish, if the pond isn't frozen over I may get time to swap it out and get a picture.

Unfortunately there isn't any ice on the pond now, or if there is, it's only a skim coating - there wasn't any ice on it yesterday, don't know about today.

Are you running the Vertex disks, if not, which ones, and how many CFM are you running through each disk now?

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).