Black Slime on pond liner walls

[reblle]

Hi I've had my pond for several years its semicircler and measures about 2.5 mts long x 1.5 mts width and about 1.5mts deep with a Laguna Pressure Flo unit with integrated UVC unit.There are no plants in the pond.There is a rubber liner fitted in the the pond
Over the last couple of seasons I have lost most of my fish apart from two goldfish which have been there from the beginning.In the past there has been about ten fish of various sizes including six cheap koi up to about 20cms in length.All seemed fine for two three seasons then gradually the koi died off.The symptoms were,hardly moving and staying at the bottom of pond and eventually laying on their side before dying , the water is clear, the fish were fed once a day, in summer, and they seemed to eat all that was given to them.
We are just coming into Spring now in the UK and I have noticed a black slime on the walls of the liner and I have just removed one of the smaller goldfish which has survived since the end of last Summer,and I have just changed about two thirds of the water.
Any advice would be welcome.

 

[JRS]

Welcome to the forum.

Your black slime sounds like an algae variety, not necessarily bad. Really not enough info to tell you anything for sure. Had you done any water testing during the problems? Clear water does not always mean clean water in terms of dissolved wastes such as ammonia and nitrites.

When you say you just removed one of the goldfish-because it died? Could have been a pathogen that spread among the fish.

 

[GBBUDD]

Usually black is a sign of imbalance. Test your water and make sure to test for kh and gh

 

[j.w]

@reblle

 

[Lisak1]

You have a very small pond for koi - I would avoid them in the future and stick to goldfish.

As for the slime - you've created an ecosystem that needs three elements: water, fish (waste) and plants. You're missing plants - which would help with the slime problem and likely the fish dying problem as well.

A pond with fish and no plants - and most especially too many fish or too much bioload - can easily become toxic to the fish because there's nothing to take up the waste they produce. Nature is going to take over and fill the breach, either in a good way - algae - or a bad way - black slime.

Is there a reason for no plants in the pond?

 

[addy1]

Welcome to the forum!

It does sound like the pond became unbalanced. A test kit can help.