I had 8 Congo Tetra in my 55 gallon and as of yesterday I only have 7. I have no clue what happened to the eighth one. He is nowhere to be found. Is it possible he got sick and died and was eaten by his fellow Congos? If that is what happened, I would expect to see at least some remnants of the dead fish. Unless maybe the assassin snails cleaned up whatever remained? I don't see any other logical explanation.