I dosed the tank with some ferts with copper sulphate in it to thin out the numbers of snails. Turns out I overdosed it and killed 2 lemon tetra, 3 black neon tetra, 3 cardinals and 3 native shrimp. I've got 3 cardinals and 1 shrimp left.
The lounge tank - long thin one with plants. It had been my storage tank for stray tetra since the cacatouides pair died.
Thanks for the tip, I'll go for a drive to Cornwall Park on Friday.