Thread: 12000 ltr pond advice needed

I am after some advice, I am trying to help a lady that lost her husband 7 months ago.
She has a 12000 ltr pond and has a kockney koi 5000v it is gravity fed, it was running a 6000ltr pump but that stopped working and her friend fitted a 16000ltr.
I have been to try and sort it out for her.
1 The water is as green as pea soup.
2 The bottom is full of fish excrement and slit ect.
3 It has blanket weed.
It is a bloody mess to be honest
I am no expert I have a few koi myself but not on that scale.
I have told her it needs total emptying a cleaning out and start again.
I have renewed the uv bulb because it has not been changed in 3 years.
I have treated for blanket weed.
I will treat it for the green algae when the treatment as been delivered.
Question 1
Do you think a 16000ltr pump is to big for a kockney koi 5000v? On a 12000ltr pond.
There are no shut off ball valves on on the pipe work so can't balance it out using them.
There is only a slide valve on the bottom drain coming from pond
I have tried to balance 5000v filter just by using the bottom drain slide valve but, the pump either cut off because it's starving of water or the 5000v over flows.
Any advice would be welcome.
My thoughts on this is to totally drain clean and refill but she is reluctant to do this because she is on a water meter.

Cheers for the reply, I talked her into emptying it and start again, she doesn't want to let them go sentimental reasons, Replaced the 16000 with a 6000ltr pump, replaced with some new filters, new uv bulb, and also said that a air pump would be a good idea.
If it had been mine, I would Replace the filter for one more suitable, I have told her it not suitable for the size of pond and that she will have to clean it regular.
There are 14 fish,13 koi and one gold fish 3 of the koi are about 10lb in weight.
It is south facing so gets alot of sun on it, there is no shade what so ever, so it's going to be hard work to keep algae and blanket weed at bay.
I have put some beneficial bacteria in, was thinking about advising her to get some alfagrog or k1 to put into two of the chambers, would that be beneficial or not? Or is there anything that would work better?
I think I will have to treat it for blanket weed and algae bloom at some point with all the direct sunlight it is getting, but hopeing the air pump will help.
I have done my best for the pond and for the lady, I would do more but it's not my money I am spending.
Any suggestions welcome