I had to euthanize Fins last night. Felt like killing.
I used clove oil to put him to sleep - then vodka to kill him.
We buried him in the garden.
In looking back over my notes on the tank (I started a notebook when I set up the tank) - it must have been the new plants I put in the beginning of Feb.
Although I did a very mild & quick bleach dip - I was thinking about snails - it's the only thing new introduced to the tank.
So I will now quarantine any plant before putting them in the tank.