Hello and greetings from Denmark.
As some of you might know -we build our schwimmteich in 2015 in Denmark.
We have had clear water from start and are very happy with that - but after 2 years we experienced big problems with algea growth in the gravel of our cleaning zone.
(see pictures of the lake in my profile).
We have done some analysing of our water and different types of gravel that we have used.
In short: the cleaning zones are build up by a distribution layer of 16/32mm washed gravel which is normally digged up from the North Sea.
In that layer the drainage hoses are placed and the water pumped through and up.
On top of that we have 60cm. of 5/8mm. heat treated quartz gravel.
The water level is approx. 50cm. above the gravel layer.
The analyses showed us that the 16/32mm. gravel (which is also used to form the edges of the lake) is releasing phosphat.
Not a lot - but enough to make the faden-algen happy and to let them grow continuously during the whole season.
As mentioned - the water is crystal clear with several meters of visibility.
Does any of you have experience that is similar to our problem?
If so - what did you do to help?
The easy answer would be to change the "north-sea gravel" to 16/32mm. heated quartz gravel - but the work load of that "little job" is extreme!
How long should we expect the gravel to keep on releasing phosphat? 2 years, 10 years, forever??
We hope to get your input - thank you all in advance.
(Replies in German language are just fine -but if I write in German, you will have a hard time getting the point )