Sorry to hear.
So your walking catfish is 20 cm (8") in 600 liters (~120 gal). Alone.
What is 0,0,10?
Zero ppm ammonia by a liquid test? What kind of test? API?
Zero ppm nitrite by a strip / dip stick test?
Ten ppm nitrate by a strip test?
I asked you to follow the protocol to help you. You wouldn't budge, it seems. Your choice anyway.
Don't trust the strips. Get you water measured by an LFS or a friend by an all liquid test tube test. Or best of all, buy an all liquid reputable test such as API:
https://www.chewy.com/api-freshwate...i23jcAu4xqs6kAyjIO4aAruYEALw_wcB&gclsrc=aw.ds
Do you have other fish, perhaps in other tanks, same water. Any issues?
How do you treat the tap water before adding it to the tank?
Is the aeration adequate?
Is the temp and pH stable?
What is the water hardness and is it stable (enough of KH would indicate the pH is stable)?
What do you feed it? Live feeder fish? Is there any other plausible source of infections / pathogens?
What's the lighting like?
Is your tap water potable?