Thanks for the response.
See I started out responsibly, then did something silly.
The readings have been fine, but i've been very methodical with water changes since adding the Tetras. Does anyone have a recommendation for a powerful filter for my tank? Would changing filter media be easy? Currently have Interpet PF2.
I didn't ask for 13 tetras, I asked for 9, and somehow the guy put 13 in the bag - I originally counted 11 when I first got home. So it's been a comedy of errors, but realise 9 would also have been too many.
So I need to look at a 2nd tank, or a larger tank now if the filter upgrade isn't going to work as I don't really have much chance of re-homing the fish to be honest.