Pond Boss Forums AERATION Should I aerate? I Just Killed a bunch of Fish

I assume you placed the air hose in fairly deep water.

One must be careful and patient when starting up an aeration system for the first time in a pond that has not been aerated for a while; this is especially true when aerating for the first time ever in an older pond. An old pond that has had decades to build up organic debris (leaves, pond plants, dead fish, uneaten fish feed, etc.) can have a large amount of anaerobic decay products built up in the bottom muck and in the lower water layers. Starting an aeration system too aggressively can pull this nasty decay products (including methane and hydrogen sulfide) up into the water where the fish are living with dire consequences.

It is also possible your air hose stirred up the bottom muck itself (which would probably been visible as dirty water), which may have released anaerobic decay by-products even faster than the above scenario.

In ponds like this (even in ponds beginning Spring aeration after taking the Winter off) aeration systems should be started up very slowly; just a few minutes per day, with very gradual increases in the length of time you aerate. The older and less aerated the pond has been, the more careful you should be.