In article , JBL2527.13138639
@gardenbanter.co.uk says...

Always had a garden pond with goldfish/grass carp, water circulation and
plants etc.
Last autumn we moved to another house. There is a well established, well
built pond, circular, approx 3m diameter and over 1.5m deep. There are
just plants in it. No shelving sides just straight down !.
There must be quite a lot of fermenting vegetation at the bottom as it
bubbles away in warmer weather.

If you mean fine lines of tiny bubbles rising, that's the plants
oxygenating the water.

No fish.
I like fish. Not an expert, I just like to see them, just ordinary
goldfish.
I rather think that I must improve the water quality and oxygen content
for fish to survive so?..
1. Oxygenating plants. I can get a variety but do I let them float on
the top or sink them to the bottom ?
2. Water circulation. Outdoor power is available so I can put in a pump.
This could power a waterfall feature ( had one before and plenty of
stone available to build one) plus a fountain of some sort.
Would these measures enable fish to survive or do I need to do more ?
If this is OK then how long before I can put some fish in ?
Any ideas or suggestions gratefully received.
Thanks all.
Brian.

I'd buy a couple of the smallest size and see how they do.

Our 6 yr old pond is the same size (also with vertical
walls,designed to deter dog). It has no pump or water circulation but
goldfish thrive in it. When we made it I put in two small ones about
3" long and the baby they had produced in their previous tank. The
originals are now 8 " long and they (and their children) breed
profusely. I give away scores of baby fish every year, trying to keep
the resident population down to about a dozen adults. We also have a
breeding population of frogs, but I suspect many tadpoles are eaten by
the larger fish. I feed the fish pellets too.

The pond plants are waterlilies and canadian pondweed; that
multiplies profusely too and since the water remains clear and the dozen
adult fish in sparkling health I suppose the weed (rooted in the bottom
of the pond) is keeping up with converting fish poo to oxygen. Keeping
plenty of plant cover gives fish somewhere to hide from herons.

Once a year I pull out a huge amount of pondweed (useful as garden
mulch) and pump out most of the water, leaving a foot for the fish, and
refill it. That's all the maintenance the pond gets.

Goldfish colouring is a mystery. Although I started with two orange
fish their children are in various colours, red, lemon, white, and my
special pet hatched last year, which is white with a red patch. (The
babies all start off dark brown).

Janet

Janet.