Oh no! That's awful, I'm sorry about what happened
What are you treating them with right now?
The best thing to do would be to forget your nitrogen cycle and just nuke the tank with bleach water since that is a sure-fire kill. If you try other things it may not work...plus some parasites' eggs can actually survive meds and even hibernate when they dry up.
Add about one part bleach to every 12 parts water and run the tank, filter, and all equipment you used on the tank inside. Do this for a few hours then rinse rinse rinse, far past the stage where you can't smell the bleach anymore. Let everything dry in the sun. When you set it back up again, if you have any doubt that the bleach is not totally gone, add 10x the amount of water conditioner after filling it.
You probably want to boil any porous or painted things instead of bleaching (like bio-media, gravel, ornaments, etc) Boil in a pot for 45 minutes. Remember not to boil anything plastic
Do you have live plants?