The water tray didn't help.

All five are dead and I still am not sure what killed them. I tried to be extra careful and read everything I could find. They were hard-to-get C99. I have five left, but don't know what to do differently.

They were all doing fine under the blue LED lamp and seemed to start struggling with the addition of a third lamp if that has any relevance.

Too much light too soon?

Use spring water instead of hard tap water?

Get a PH meter/tester?

Get a different hobby?