If you must aerate an old pond in which the bottom is covered with deep sediment which has become anaerobic, or devoid of oxygen, exercise caution. Quite by mistake, you might start a devastating fish-kill. Fish ponds are not the only impoundments that carry the potential for trouble.
Small ponds covering one surface acre or less can be especially vulnerable.
He installed an electric aerator and three days later, found 80 fish floating including catfish that weighed 10 pounds apiece. To tell if you have a potentially dangerous situation for your fish, stir up the bottom sediment and smell the rising bubbles. Be alert for a distinct sulfur odor. The offensive odor is hydrogen sulfide, or sometimes methane, both products of anaerobic bacteria which develops as a byproduct when vegetation and other living matter decays. Both gases can be deadly to fish and people, so be careful.