To most Koi keepers they associate ulcer break outs with temperature. I feel that this a misconception, based on my experience over the last 20 yrs working with ponds and Aeromonas break outs. I will explain my experience and what I feel is the cause of the out breaks.

First you have to understand that Aeromonas is always in the water and always present. I use to think the same thing that it only came out at a certain temperature and then would attack your Koi when there immune systems were the weakest. Over time seeing this bacterium in action I come understand it only to find that temperature was not the factor involved here. It is more with the Koi coming off wintering then temperature based; lets look at why this might be.

What is happening is your Koi have just come from a winter period and are weakened by cold and fasting, which depends on the type/length of the winter for your area. The effects of this cold and fasting, causing STRESS thus leaves the Koi in your pond wide open for bacteria to attack. Remember this fact that Aeromonas is always in the water and always present. So adding the post winter effects; stress/exhaustion and add in bacteria to the mix, you can see where this term "Aeromonas Ally" came about. When in reality it is the combination of more then one factor not temperature.