Also posted to the UK group. Apologies for anyone reading twice.

After two power outages of about 3/4 and 1 hour, of my three tanks the
smallest (Juwel Rekord 60 with plants and 12 Platys) reads perfect water
quality.

The Juwel Rio 300 (plants, Platys, 4 Clown Loach, 6 Angels, 6 Danios, and 4
Corys) is fine except for the pH.
It's dropped to 6.0-6.5

My Rio 400 (Plants, Comets, Shubunkins, SAE, Tench, Orfe) readings were dire
!
Ammonia was high (4.0), pH had fallen to 6.0, Nitrite was 0.0, and Nitrate
was 40.
After a 40% water change pH was still 6.0, Nitrite 0.0, Nitrate dropped to
10, Ammonia 2.0.
I've now double dosed with Prime, so will have to wait 24 hours to read it
again.
Is the worst over, or are my fish doomed ?

As for the Rio 300, does it need a waterchange ?
The 2 smaller tanks have a 25% waterchange weekly (last Wednesday), and the
big one fortnightly (also last wednesday)

Peter