Current theory, after finding a slug/snail trail leading into the tank, is that a local slug fell into the tank and died.
I did stir the substrate up, but it's gravel and not too thick, so not too worried about gas pockets. It's vac'd every week and to be honest wasn't too bad at the weekend.
Nitrite now risen slightly with ammonia between 0 and 0.1. Nitrates are now at 20. Another water change this morning and I'm hoping that we're soon to be clear as the filter seems to be picking it up nicely.
I did stir the substrate up, but it's gravel and not too thick, so not too worried about gas pockets. It's vac'd every week and to be honest wasn't too bad at the weekend.
Nitrite now risen slightly with ammonia between 0 and 0.1. Nitrates are now at 20. Another water change this morning and I'm hoping that we're soon to be clear as the filter seems to be picking it up nicely.