All stats absolutely fine, although I don't test for O2.
The water change has helped dramatically - I'll keep doing 10% changes per day.
I try to ensure the water quality is at it's best - plus there's plenty of plants (the tank is covered with anubias berteri, java fern, vallis & HP - and some floating plants) which are still thriving, and help the fluval 405 with the filtration which has held the same bacteria colony for 6 years.
I've been reading the scientific thread in this forum where I found that tap water contains ammonia which is only present once the stabilizers for it (which are added by the water companies) gasses out, around 12 hours or so later - maybe the large water changes I've been doing have resulted in ammonia being present overnight - then the filter dealing with it ASAP so that when I wake up and test water stats the following day it has dissipated?
However the fish have to put up with a short & small ammonia spike, which is why they may be breathing heavily? - Just a thought.
Thanks,
Paul