Been there too
I did a water change on the tank in the bedroom, and my hubby was ill. I didnt want to wake him, so I just used the light from the hallway. I didnt use a siphon on that tank that night, just a big gulp cup. as I was pouring the water down the toilet I saw a shadow of something pour out of the buckt. Didnt think much of it and kept going... filling tank back up, I did a quick headcount... 1, 2, 3, 4....... where is number 4??? *smacks forehead...* UG!!! Im so dumb.
I couldnt believe that!!! I was SOOO mad at myself, and I feel so sorry for my poor little tetra, I hope it wasnt too bad... at least yours didnt die in raw sewage.
I can truely say I know how you feel.