Are there any valves on the filter ?
Sounds like it may be in backwash mode and pumping to drain.
20k ltrs of water wouldnt be down to evaporation so your pump is leaking as you say it only happens when it’s running.
Does it have a fountain or something thats aiming over the side?
Pump is leaking? How can a pump leak?
Yes it has a downslide with some welsh stones running fown two paths
Maybe some of the seal here is broken? Then the water goes into the ground while it trickles down the path?
Are there any valves on the filter ?
Sounds like it may be in backwash mode and pumping to drain.
I see all the threads have been merged.
The koi have uprooted the moved iris. With the sun the blanket weed has been going nuts but joe the veggie filter is in in full growth the blanket weed has slowed.
I had a major problem with blanket weed and a massive invasion of Hornwort...tbh I never noticed the hornwort until my pond suddenly grew a Cotton wool beard all along the bottom and then the top..I used some colder leaf and removed what I could but in the end the only solution was to remove it all manually...I’m nearly there...
Hornwort is actually a really good plant to have in your pond as it forces out algee