Wildlife pond that has gone stagnant/smelly

[blueherb777]

We dug out a hole and put in a ready made hard liner pond (about 4ft long)and deeper at one end about 2 months ago.
So it could become a Wildlife pond.
We filled it and put some oxygenating weed in the bottom and lilly pad plants in pots on the shelves either end (all doing OK).
We had some algae form on the pond sides and also put some daphnia in.
A couple of weeks or so ago we saw a small newt just in the edge of the pond.

But the problem is we noticed the water become dark looking and about three or four days ago it started to really smell stagnant.

I went to a Garden Centre today and he said something could have got in the pond and drowned.
So I took a lot of the water out and put a net in the bottom but could find nothing ?
The net bought up a lot of smelly mud/gunge from the bottom.
As I took 90% of the water out I filled with fresh water.

Does anyone have any ideas why it has gone stagnant/smelly ?
And what we can do about it.

p.s.

When I took the two pots out from one corner and put back again something jumped off one of them into the water (a frog).
It is the first I have seen in the pond (does not seem to mind the water conditions)?

[Kayleigh]

My barrel did this I took all the plants and water out rinsed the plants with a hose as they were all black and smelly then put them back and refilled it with tap water as this is all was available.
Touch wood its been fine since.
Sometimes the rotting leaves or dead plants can turn the water like this.
Just get as much out and change the water add more oxygenators and yellow flag iris as they act as a water filter..

[ollyk]

Yes I wonder if you put too many plants in an immature pond without enough nutrients to grow - hence plants rotting = ammonia and lots of algae?

That is pretty much how it works with planted aquariums at least!