Nem died and you still need to clear the water regardless of test kits.
Watch your animals....and don't worry about the worms.
Are your fish at the top gasping for air? If not then there is not enough free floating ammonia in your water to kill your fish. As others stated API is terrible and don't rely on the readings.
Do you have carbon you can run to help pull out some of the nasties the nem released?
With a year old tank a good size water change would be good but definitely source your water from a LFS so you know it's parameters are on.
Sounds like your bacteria is working to clear the bad waste. The carbon will help with that.
Until your fish are freaking out you don't need to freak out