:(Herb committed suicide :(
Realised I couldn't see him in the tank a day or two ago, and not really many places to hide in there so couldn't work out where he had ended up, until I saw him in the back compartment. Clearly climbed out and 'drown' himself.
As much as I hated the little fucker from the time he ate all of my snails, I'd still had him for a year and sorry he went that way.