Hi,
Jens Christian, are your plans growing well? Yellow leaves and growing slow? My algae developed in the same way. In the first two years, they were like hair, strong and sticking together with the gravel difficult to rib them up. That changed over the last 3 years, now they are like yours.
Well I have Problems with decreasing minerals and makro nutriants, the water is getting softer. Yellow leaves, bad growing plants, getting worse from year to year. A professional lab Analysis of the water last year showed: no Phosphate, no Nitrate, but same with K, Fe, Ca, Mg - Impossible for plants to grow rapidly and healthfully.
For this season, I changed some things: I am constantly filling up the water loss with tap water instead of rain to harden the water and I started to fertilize my Pond in small doses with KNO3.
One theory I read about is, that the reduction of Phosphate by plants and bacteria is interfered by the lack of nitrate. Normally the amount of Nitrate does not increase the growing of algae since the system is limited by Phosphate. To reduce Phosphate constanly when it occurs, you need nitrate for the metabolic process.
At the Moment, I put two doses of 5mg/l NO3 in the water. The difference in plant growing is huge! The leaves of some plants start to change their colour from yellow/light green to green/dark green. No increasing amount of algae so far. It is too early for a final conclusion, but things are getting in the correct direction! I am very excited and Keep you informed...
Greetings
Manu