Angela; there's nothing wrong with using an aerator (at the proper height) AND a breather AND a cattle trough heater AND a pump near the surface AND a heater AND a cover; they all can be used and will give you better odds for fish survival. I just don't think they're all necessary in lower Michigan, as some TOP EXPERTS think. If you have enough volume and depth, have a balanced pond (re the water parameters and established biofilm) have a fish load commensurate with your pond size, keep the surface open so light can penetrate (one major component you didn't have this winter), have a hole open for any toxic gases (from ongoing decay) to escape, and don't have a lot of decaying matter during winter months, you'll be fine here in MICHIGAN; I won't speak for Canada--that's for top experts. Any of the above can derail success AS WELL AS some fish are just more sensitive to extremes; be it O2 level, CO2, temperatures, disease, etc.
Since I have the same weather, more or less, as you, and have lost maybe 5 fish in 7 years during the winter, seems I might have an insight.
If your fish are at least 10 years old (as you state in an earlier post), I’m surprised the largest is ONLY 12”? Thought koi got a lot larger than that (I have 12” goldfish!)? So your case is NOT one of large koi in the dead of Siberia, and not even close to what I have (100 gf, most over 6”, 10 or so at 12”) in 2700 gallons. I had an aerator ‘Fail’ one year and possibly lost 1-2 fish that year. That’s not why I got the breather though, I just felt it would ‘fail’ less than another aerator. I’d still use one if my breather went out, but would have to melt a hole in the ice. A dual system is a good thing, but I don’t think your fish died because of lack of oxygen DUE TO THE BREATHER; a breather does NOT add oxygen, nor does an aerator, but an aerator does facilitate the process better than a breather because of the turbulence. But this turbulence goes down as ice closes the aeration hole with slush. In the dead of winter, unless you have a large air pump and can maintain healthy turbulence, an aerator will only do what a breather does; keep a hole open to release any buildup of toxic gas.
btw, did we establish exactly how much volume you have at the 4.5' mark? I actually think you have more than 2100 gallons. I think you mentioned your deep part begins half the length of your pond, so that would make it 10' x 10' x 4.5' (7.48) and give you at least 3300 gallons. Which is a lot better than thinking you have half your 2100 per the counter. I'd get some sort of better estimate, just because you might need it one day for possible dosing (disease, pond conditioning, etc).
I think it would help knowing exactly how much volume you have at the deepest depth because as noted before, the more you have, the more capacity for holding O2. And it looks like the lack of is what probably did in your two koi. From the data you’ve already given though, I don’t see depth nor volume per your fish load as being problematic. You had (4) 12” koi, a gf at 8” and the black molly (I’m assuming another koi?(not the tropical variety). That’s not a heavy fish load unless your koi are a lot larger than 12”.
Michael
Like Mitch, I use a membrane disc aerator, it's for my 40 LPM ( Matala is the brand ) aerator I run in the summer months. I run a 20 LPM Pondmaster aerator in the winter and cool parts of Fall and Spring....it has 4 airstones.
