OK I need some MFK advice. I am almost positive now that it was bleach after thinking about it. I went into the laundry room and removed the whites from the wash machine. It still smelled of bleach. I took the clothes out of the drier and put them away, then stuck the python in the tank. I definitely got my hands wet. I didn't really consider the possibility that the bleach wouldn't be rinsed clean in the washer. Symptoms: More than 50% death in less than an hour. Heavy fast breathing, over activated slime coat.
I'm assuming it is bleach. I did 3 70% water changes on the tank. I removed all living fish to teh quarantine tank. Scaled fish seemed pretty much unaffected. Only loaches died. The last fish is a larger pleco that was never removed. I'm going to do a 90% water change tonight and thoroughly vacuum the substrate. I've had a large carbon pack in the filter for 24 hours. Any other suggestions? The remaining loaches can't stay in the quarantine tank too long, it's too small. I need to make the tank safe again.