• 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.

    The questions:

    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 ;))

    Best regards

    Jens Christian

  • Hi there,

    the problem had already been announced last year. There are several ways to bind phosphate. Firstly, the phosphate binder block (available in the shop) for long-term phosphate binding (highly recommended). Then there is "Phosphate minus", a powder that is sprinkled into the swimming area and binds phosphate there for weeks. The encore is more interesting for the autumn monsters, so that it can work over winter. Must then be suctioned off. The last option is turbo phosphate binder based on lanthanum. This helps visibly and measurably immediately. But should e.g. be dosed at an interval of 2 weeks since it can have an impact on pH and thus on biology. Otherwise completely non-toxic.

  • Hello Kotzbrocken -and thank you for your reply.

    You are of course right that we have written about the problems before. And we have tried several of the advices that we have gotten -also the phosphate binding blocks.

    But with no luck in lowering our algea growth in the cleaning zones.

    After that we had some more tests done at a danish laboratory -both tested the water of course, but also the different gravel types we have in the lake.

    Here it became more clear that the key to our problem is most likely our 16/32mm gravel (from the north sea).

    It is slowly emitting phosphat. Not much, but just enough that the faden-algen keeps growing and growing all season.

    So the question is: what has other people done in that situation?

    Trying to lower the phosphat level with the block or phosphat minus will (in best situation) only help in very short term as the gravel will continue to "produce" phosphat.

    Are there any other solutions than "starting over" and taking out all the gravel (120 ton+) ?

    Thanks again :-)

    Jens Christian

  • Hi kotzbrocken - thanks again! :-)

    When I make my own readings with the measuring instrument I bought from TopTeich shop I get readings close to "0". I haven't made scheduled test over a longer period, but it seems stable and low.

    But when we had the different gravel tested at a laboratory the 16/32mm gravel showed 0,4 mg/L P !

    -and much less in the 5/8 filter gravel and almost nothing in the "free water".

    I attach the analysis report (in Danish, but I hope you get the facts out of it...?


    He also measured the filter gravels oxygen consumption over 24 days.

    That was very low -indicating that the bacterial culture is weak...

    But again: we have crystal clear water all year long, so something is really working...

    It is driving us a bit crazy -any input and suggestions are welcome! :-)


    Jens Christian

  • The test appears to be somewhat inaccurate. A value of <0.1 mg / l says nothing precise. A value of less than 0.035 mg / l should be aimed for. It looks like a droplet test in the photo that does not achieve a usable result. The value in the gravel can be bound phosphate, so that would be ok. How bad does your pond look like? Maybe you have photos from last summer. Guess that your expectations or expectations of a natural pool are too high. A certain amount of algae and mulch is normal. Pure nature !

  • That is a little more. If you are already using the phosphate binder block, you could try the Turbo Phosphate Binder. I have had good experiences with this because it reacts at lightning speed and deprives the available algae of the nutrient supply. Please do not dose too high and repeat at least 14 days apart.

    I don't see any other options at first.

  • Definitely no surface water - that is not possible.

    Rain of course - but no surface water running into the lake.

    Load on the refill water? What do you mean?

    I have tested the refill water (tap water/city water) and that is high in phosphat, but not extreme.

    But the 250m3 that filled the teich from start was the same and we see no difference in periods where we refill (summer and hot) and periods when we do not - all spring and fall typically. Same algea growth most season.

    But the fact that the first year (october til december) and all second year we had NO algea growth at all is also what confuses me. If the 16/32 gravel is the source of phosphate -then why not from the start?

    I really appreciate your input!! Thanks! :-)

  • Hi Jens Christian,

    ich schreib dir lieber auf Deutsch my Englisch is terribl ^^.

    Einen tollen Teich hast du da gebaut , das mit denn Algen ist natürlich nicht schön , vor allem die viele Arbeit dieses Zeug wieder zu entfernen.

    Ich bin zwar noch Neuling in Sachen Schwimmteich versuche dir aber evtl. paar Anregungen zu geben was mir so auffällt an deinem Teich .

    1. Würdest du nochmal deinen genauen Filteraufbau beschreiben ? Ich hab nichts im Forum gefunden , nur das du anscheinend über Rundskimmer die Wasseroberfläche abziehst und du 3 biodrain damit betreibst. Wird dazwischen nochmal feingefiltert ? z.b. Bogensieb ,Druckfilter ..... oder leitest du das Wasser direkt zu deinen biodrain ?

    Laufen deine pumpen 24 Stunden 7 Tage die Woche durch?

    2. Hast du schon diesen Phosphatbindeblock im Einsatz? Solltest du in jedem biodrain einsetzen .

    3. Du solltest wenn du schon einen 1 Topf Schwimmteich hast mehr Pflanzen einsetzten, um Phosphat und andere Nährstoffe in Blattmasse umwandeln . Diese Nährstoffe stehen dann denn Algen nichtmehr zur Verfügung.

    4. Du hast viel Flachwasser wo auch die Algen wachsen . Ich würde stellenweise denn Kies über Wasserlinie aufschütten .

    5. Algen brauchen zum wachsen Licht und Nährstoffe, durch mehr Pflanzen hast du mehr Beschattung und weniger Nährstoffe .

    6. Algen entstehen meist auch bei zu wenig Sauerstoff im Wasser und produzieren diesen dann . Sauerstoff bekommst du sehr gut über ein Bogensiebfilter ins Wasser oder über einen Wasserfall . Ist bei dir im Teich etwas vorhanden wo Sauerstoff ins Wasser einbringt ?

    7. Wie sehen deine wasserwerte ansonsten aus ? Ph, Gh, Kh ....

    Ich hoffe dich bringt das ein bisschen weiter und du bekommst das verständlich übersetzt .

    Beste Grüße Timo

  • Hello Timo - thank you for your reply and thoughts! :-)

    (..the German is just fine!! :-)

    1. Sure, between the rundskimmer and the biodrain sits the TopTeich filters with brushes and a "mat" of blue material. So the filtration should be ok.

    Pump is running 24/7 around 9 months pr. year (shut down and taken in from end December till mid March normally).

    2) The Phosphatbinderblock has been in the cirkulation from around summer 2019 till pump shut down.

    It has been sitting in the filter-box where there is a high flow of water.

    But we did not see any difference in the algea growth.

    3) Plants is a good point. But our experience is mixed with the plants. Some can grow -but most will not survive for long. So our conclusion is that there is not much nutrition for the plants. So I am not sure if a lot of plants will make a positive difference. But it would make cleaning out the algeas a lot (!) more difficult.

    4+5) If we would build the schwimmteich today, then we would have made two separate ponds (swimming/cleaning) and probably have the gravel above the water line in the cleaning zone.

    But changing that in our lake, as it is now, will be a very big job... But maybe we will end there?

    6) Sauerstof - I translate that to Oxygen (right?). According to a danish water biologist - when we have clear waters as we do, with "sichtbarkheit of 10+ meters, then the water is gesättigt with Oxygen.
    But you are right that we do not have any waterfalls or similar to "mix up" the water. I am not sure if it would do any difference for us?

    7) I don't have accurate readings of other than the Phosphate level. Again mostly because the danish biologist claims that the clear water it self is "proff enough" that these levels are fine.


    I am not sure. A lot of plants could be one way to go - but we fear that the 16/32 gravel in our lake will keep on emitting Phosphate enough that the algea growth will never end....

    Thanks again and greetings from Denmark :-)

    Jens Christian

  • Hi Jens ,

    Ich würde auf alle Fälle denn Phosphatbindeblock weiterhin benutzen in deiner Filterbox. Es braucht schon etwas Zeit und ist auf alle Fälle nicht falsch . Auch würde ich wie von Kotzbrocken schon empfohlen mal denn Turbo Phosphatbinder einsetzten.

    Wegen denn Pflanzen würde ich aber trotzdem versuchen einen Bereich wo ein biodrain ist denn Kies über Wasserlinie zu ziehen und Schilf ( Phragmites australis) pflanzen , das ist relativ anspruchslos und wird auch in großen Reinigungsanlagen gerne eingesetzt. Ich glaube es ist einfacher regelmäßig diese Pflanzen zu entfernen als Algen .

    Deine Wasserwerte kannst du gut mit denn Mehrfach Streifentest hier aus dem Shop prüfen . Ich würde dir dringend raten deinen Ph , Gh und Kh zu testen egal was dein Wasserbiologe sagt . Ist dein Gh und Kh Wert zu niedrig kommt es gerne zur algenbildung und zu Schwankenden Ph Werten. Das bringt die ganze Biologie durcheinander in deinem Teich . Währe schön wenn du mal selbst die Werte testen würdest mit denn Streifen , das kostet nicht viel und man kann ein Problem mehr ausschließen.

    Hier mal noch eine gute Seite wo wasserwerte und der Zusammenhang gut und leicht verständlich erklärt wird . Die Seite wurde hier im Forum schonmal gepostet darum denke ich das geht in Ordnung.


    Grüsse Timo

  • Hello again -we are back from vacation and getting ready for spring :-)

    You both recommend the Turbo Phosphatbinder from the shop.

    But when I read the description it states that it will lower the phosphat to about 1mg/l -but that is still very high! My phosphat readings are normally much lower -but again - our 16/32mm gravel seams to release phosphat at slow steady pace....

    What to do?

    Also: since we have 250.000l water - I would be using 3 x 5l Turbo Phosphatbinder for each application... it will be rather expensive.

    What do you think about the 1mg/l statement?
    Will it be worth a try?


    Best regards from Denmark

    Jens Christian

  • Hi, I think they were talking about the "phosphatbinder block", not the "turbo phosphat binder". The block is, well as the name says, a block, a chunck of something that you put in a net and hang it somewhere into the flow. So it continously "treats" the water. The thing will last, depending on the flow speed at that location, several month. Usually people use only half a block, since one whole block is supposed to be sufficient for ponds with up to 400.000 L of water. For your case maybe you should use a whole one. It cost 149 EUR here in the shop.

  • Hi there,

    the Turbo Phosphate Binder binds up to 1 mg / l, i.e. a maximum of 100 percent. You first need 5 l, which you divide into two.

    Then be sure to buy the Hanna photometer and test strips to check your values yourself. Phosphate, pH value, total hardness and carbonate hardness. You can then continue working with the values.

  • Hi kotzbrocken - thanks, but I have to admit that I don't understand the Turbo Phosphate Binder.

    As I read your text - it's maximum abillity to lower the phosphat level is 1mg/liter x 5 liters = 5mg in total.


    Why the "divide by two"?

    Sorry for my lack of knowledge here :-)

    I have the Hanna photometer - and I will order the test strips when I am sure if I should order the Turbo Binder as well -and how much :-)

    Thanks again

    Jens Christian