"Bengt Boysen" wrote
I would also suggest that you completely drain the pond, remove excess
mud,
invest in new plants. Then you fill the pond again. Then you should add
one
or perhaps two bales of straw to the pond. This will keep the pond free
from
algae. Then, as some other has proposed, buy a pump to circulate the
water.

This is a clay lined pond, if it's drained then the clay will dry out and
crack, result, relining required. Removing excess detritus from a clay pond
is also fraught with danger, where does the mud stop and the clay begin.
As I said before , ID your fish and test your water with a proper test kit
(about £20 and half an hour). You will then know the extent of your problem.

If it's carp in the pond you will need to decide if you want a permanently
muddy pond or to get rid of them and stock with something less destructive
like Golden Rudd, Golden Orf etc. A local angling club would probably be
glad to remove the carp to stock their own waters. But whatever you do
remember not to disturb that clay lining.

--
Regards
Bob
In Runnymede, 17 miles West of London