In general, the best pond aeration solution for any pond is simply to get it aerated! There’s no doubt about the host of very beneficial processes that a properly aerated pond brings to the ecosystem it thrives in. However, there’s one- and really only one- time you need to act with caution towards aerating, and that’s wen dealing with an old pond that’s gone un-aerated for a long time.
The older pond will have already lapsed into that anaerobic state that the act of aeration hopes to avoid. That anaerobic matter will be an issue if you suddenly and without warning aerate the pond, and can lead to devastating fish kills and more. It may not have been a healthy system, but it was a system- and you’ve just turned it on end abruptly and without warning. You will, in the aeration process- if done to suddenly- stir up too much poisonous gasses and anaerobic water for fish surviving in the relatively healthy layers to tolerate.
If you’re concerned, investigate the sediment at the bottom of the pond and put a nose to the rising bubbles. An offensive odor of hydrogen sulfide and/or methane means trouble. You’ll get these in situations where anaerobic decay is taking place, and they’re both deadly to fish [and people].
For exploratory purposes, you can lower an air diffuser stone into the water. Stop at the level you pick up any hint of the sulpher smell form the bubbles it produces, and raise it to a layer where that’s not present. Know that the water underneath the stone is the water that’s saturated in the undesirable gasses and little to no oxygen. There is no fish down there, as they cannot survive in that environment. You can begin your diffusion gently from this point. The aerator will begin to remove the sulpheric gas, but at a slow rate that will not bring harm to the fish living in the healthier layers above it. Very gradually, and always adjusting to a height where there is no strong smell present, you can lower the diffuser over a period of weeks, slowly allowing the diffusor to improve the water quality without at any point overwhelming the top layers with too much of the stagnant material.
It’s equally as important, however, to use a bucket or similar shield on the diffuser once it reaches bottom depths. You also don’t want to stir up the anaerobic sediment at the bottom layer of the water, again to prevent oversaturation of the healthy water with poisonous substances. It’s laced with the very bacteria producing the noxious gasses that place your fish at risk. However, as the diffuser works, it will create an aerobic oxygen rich environment which will kill off the dangerous bacteria in the sediment layer- to be cleaned up by beneficial aerobic bacteria that will develop. Eventually, over time, the aerobic bacteria will clean up the anaerobic bacteria and freshen the water from top to bottom, at which point you will be able to carry on with installing the aeration system you want for the pond.
Remember, it’s only with gentle baby steps that you can recover an old, anaerobic pond without killing off the fish and other animals living in the water. Don’t do too much too soon.