This corner had separated,
and despite my best efforts, I couldn't stop the leak.
So I had to completely dismantle the tank, meaning containers of fish, plants and water all over the house.
After resealing the glass and allowing to the sealant to cure overnight, it was 'simply' a case of putting everything back together.
I did lose one fish, one of a pair of Angel fish
, but it could have been so much worse.
The tank now.