Another 2 cents.......
What about the environment outside the tank.
Was there anything sprayed in the air near the tank, such as air
freshners, bug sprays being sprayed in or outside the house, anyone
have anything on their hands and put them into the tank without washing
them (I did this the other day [kicks own butt], I forgot I had put
lotion on my arms as well as my hands and I forgot to wash my arms
before feeding bloodworms by hand. Luckily I got my arm out of there
fast enough and I performed a water change just in case). It can
sometimes be things so obvious, yet easily missed by some.
Another thing...what was the temp?