I feel so stupid...
I knew ahead of time that the methlyene blue would kill off the beneficial bacteria. An ammonia spike was inevitable.
I dug through a box of old aquarium stuff an found an old test kit. Ammonia was literally off the charts. As I was testing, I lost another koi. He died in my hands...
I moved the remaining 2 koi to a de-tox tank and began draining the other tank. I changed out 65 of the 75 gallons and the ammonia was still at 2 - 4 ppm.
The tank I moved the koi to had 50% new water and I'm changing out 25% of it hourly until the ammonia reaches a reasonable level.
I'm so mad at myself. I should have been testing. I started with 8 koi in the garage, and now I'm down to 2.