Sorry about the late reply, the only testers i have are for ph and No2. Was told the other testers were more for marine tanks ? i hve 6 Harlequins, 6 Neons, 2 frogs and 3 guppies, now down to One.
Done two big water changes (70%) then the second one got the same symptoms and died, None of the other inhabitants are off the food or look ill.
Tank is 90 Litres , Have live plants in, Vary the diet and do my water changes regularily.
Dunno what else to add.
Cheers for the comments