Hi Chris, according to Collin the Seponver plus does not work and he also tried Dimmilin without success.
I have a lot of Dipterex(Trichlorfon), but it expired and if old, it turns in Trichlorfos which might kill the fish. Trichlorfon may not be sold anymore in South Africa for the last three years, and all stock available are old stock.
Maybe you should try Copper Sulphate.
It is seldom a problem in the pond itself. It is normally a problem in the filters and block the piping. You could also empty your filter, and sprinkle a few cups of course salt over them, leave it for 10-15 minutes, and open the filters again(before your good bacteria die)