I was hoping some others would join in on your post...so if the floating Koi was what brought your Nitrites levels too high, than what killed off your three Koi? I do think that you need to invest into an air stone to bring more oxygen to your fish and stop feeding the fish until the levels drop below 1.0 and aerate as much as possible. You can also add salt to further protect your fish's health.

Have you read Aquaponics 101 by Oliver? You can find it at the bottom of the Forum Page.