Big quaterly cleaning process of the inner filtres today... and big water change to follow... a bit more than usual... 3,500 liters, which is 35% of the pond. I lost 4 to 5 celsius doing this.
after water change, I discovered my pump on filter 1 is producing quite a big noise... it must be sand in the rotor, or maybe an air bubble somewhere... I'll check that this weekend.