There are many things that can compromise the natural immunity in our Koi .. but generally Koi are quite hardy fish and have an exceptional natural immune systems which can, in many cases, heal wounds without the use of antibiotics. The first line of attack for the bacteria will be the skin or the gills. Their natural immune system is compromised by a whole host of things, including , but not limited to:
Shipping: when you receive your Koi by special carrier or you collect directly from your local dealer or wholesaler, they will be weak, not just from the actual journey, and usually not fed for at least 5 days, sometimes they may have been treated with mild medications to calm them down for travel, or if traveling during warm weather then they may be cooled down with ice to keep them as cool as possible .. the cooler the water the more oxygen is contained in that water but as the water warms up the oxygen will begin to deplete - but one thing is almost certain that they will be in Ammonia saturated water. all of these factors increase the resistance to medications and lessen the natural immunity in our Koi. And this is why quarantining new arrivals is so important, and if you do not quarantine the Koi under ideal conditions for approximately 3 weeks, then the natural immune system in our Koi has no chance to regenerate on it's own - it needs your care and assistance.
Temperature: While we may keep our Koi at less than optimum temperatures which is desirable in our ponds once acclimatized, new arrivals should be kept at 22 to 24 degrees Celsius, 71 to 75 degrees Fahrenheit for at least 3 weeks, this helps the natural immune system to regenerate slowly.
Oxygen: Ensure that you can provide the maximum water circulation as possible during this period of quarantine, for oxygen absorption and CO2 removal. It is essential that you provide a good supply of air generated by a good and reliable air pump and there are many on the market to choose fromOnce the Koi are ready to leave the quarantine tank and you place them into your pond with the other Koi and pond inhabitants .. then please keep your eye on them especially for the first week to ensure that they are behaving in the same way as the other fish... and if one of them is off on its own and probably hanging around in the corners of the pond .. then there may be a problem with that fish so the first chance you have catch the fish and carefully examine it for physical damage and gill problems .. if you do suspect gill problems then try giving it a salt bath using 7.5% solution for 10 minutes - but make sure that you place at least one air stone in the bath with the fish and never leave it unattended, you need to be there to see any signs of distress, if there are remove the fish from the batth immediately ... This treatment can be repeated every other day until the fish shows signs of improvement.