Mike has it right. Aeromonas is an omni-present creature in any pond environment. If you do a test for them and it is zero seek a new testing center!
The name of the game is to keep the count low. Not by trying to kill it off. But rather providing conditions that simply don't allow or encourage the rapid growth in numbers.
The number one case of this dynamic is over feeding in early spring. The number one case in water gardens is dirty water.
The other side of the equation of course, is the condition of the fish. Fish should be fundamentally healthing coming out of winter, but weak as they rouse. TLC is a good approach. Small meals and routine maintenance. Ironically the garden ponder that sets their fish up for 'aeromonas alley' are the ones who empty the pond and do 'spring cleaning'! They disrupt the biology, add chemicals and clean filters to the extreme. This puts fasted wintered fish ( especially OVER wintered fish who experience G.A.S. from cold stress) at a distinct disadvantage. If a fish is under ice for 10-18 weeks and the ice has now melted into the water, the very last thing they need in this very weak condition is another environmental challenge!
The 'art' of koi keeping is part of the 'science' of koi keeping at this point. Small but frequent maintenance is so very important. And small amounts of food is also key as the fish is not ready to assimilate large meals and most food will pass thru and further deteriorate already stale water conditions.
Daily sump dumping is a good start. Followed by 10% weekly water changes. Of course this all assumes you kept the filters running, which all hobbyists should do. If you bring the koi insode then start up the pond in early march and do not put the fish in until april 1. This will give the filters time to grow and mellow the new water ( In this case I WOULD do a 50% water change and dump the sumps weekly even though teh fish are not in the pond yet).
My fish ate yesterday for the fist time this year. I feed them OJ soaked Hikari wheatgerm pellets. I have 16 koi and the water is 51 F. I fed them one handful ( about a cup) of pellets. I'll likely feed them again on Friday. But this does not mean I won't put in a dozen pellets before then as I test the behavior and observe the condition of each individual as they slowly rise to the pellets.
Aeromonas alley is a valuable urban myth as it has the beginner hobbyist alert and on their toes watching for early problems. It is valuable in that it teaches the beginner, who imagines their koi cozy and sleepy from 'hibernation', to consider another reality - that the koi have survived another winter. The facts of aeromonas alley are unfortunately accurate enough to have brought older hobbyists to an incorrect conclusion. That koi have no immune system in a temperature range where aeromonas is on the rise. If that were true than nature would have made carp extinct a long time ago!
Since the emergence of the theory of aeromonas alley, higher end hobbyists have muted winter effects/condtions ( running filters but covering and/or heating ponds) and improved husbandry/filter techniques and pond designs. And the legend of Aeromonas Alley has pretty much been put to rest. Koi simply do not get infections when winter conditions are managed for an ideal winter stasis. This is because the key to all of this is the stress of winter conditions and what that stress does to a fasting, wintering fish's immune system and general condition. - JR