All my fish died in 24 hours

[MnFish1]

How are you ruling out vectored disease say back in February

if he added a clearly smelly bag of dead cuc I’d consider ammonia shock

does not run tanks with fallow by practice

but if that bag didn’t stink, and these guys add much more raw ammonia directly to reef tanks on purpose, I couldn’t see ammonia as a cause for the op:

Does anyone dose ammonia and/or how would one try?

So, after an interesting discussion on nitrate dosing and looking at my Walt Disney coral and seeing it slightly less colorful than I have seen it in others tanks, I have decided I want to learn about and then try dosing ammonia instead of buying 20 damsels. Nitrate additives I see abound and...

www.reef2reef.com

I'm not sure that there is any way to know how much ammonia would/could be added by a CUC - as compared to them all dying suddenly due to a toxin in the water - as compared to someone dosing ammonia?

In reality - I think a disease would be extremely unlikely - especially since the OP stated that he has 5 tanks and only had a problem in the tanks into which he added the CUC