Question
My large Koi and tench are constantly hanging under the waterfall in pond for the last day or so and they don't appear to be very hungary is this down to the heatwave or is there a problem?

Answer
Hello Helen!
Yes, when fish do this it indicates an oxygen problem!
The water needs to be tested immediately for ammonia and nitrite. If these toxins are present then 1/3 water changes need to be made until they return to zero (and all feeding needs to stop). Do not forget to de-chlorinate the water for each water change.
Even if ammonia or nitrite levels are zero, the water needs to have more oxygen and an air pump and/or fountain needs to be added.