Friday, February 25, 2011

DO Level

We have about five large koi and have had them for a long time. We woke up this morning and they were all dead, but all the little koi and goldfish are fine. What does this mean? -Karen

It appears that the levels of DO (dissolved oxygen) in your pond water have dropped below levels that the large Koi need to survive.  Large Koi need more oxygen than small Koi, and unfortunately, they are the first ones to go when the DO levels drop.

Why did the oxygen level go down?

As temperature of water increases, the amount of oxygen it can hold decreases.  Some types of Algae in the pond will produce oxygen during the day, but at night they will strip it from the water.  Also, if you have a waterfall that aerates the water: up until last year it could have supported your fish load just fine, but this year your fish load could have increased (due to the Koi growing), so it may have not been effective at keeping the DO level up.

Add an air pump and an air stone to your pond setup to maintain the proper oxygen level in your pond at all times.  You can also find test kits that measure the DO level in the water.

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