By "fiber pool" do you mean "fiberglass pool with a gelcoat finish"?

It's possible to remove calcium carbonate scale (CaCO3) from a fiberglass pool by lowering the pH to 6.0 - 6.4, and lowering the carbonate alkalinity to less than 80 ppm, and then holding those levels for an extended period. BUT . . . in order to do so, you need accurate testing capability and you need access to muriatic acid (~30% solution of HCl). If you were in the US or Canada, I could tell you where to go, and what to buy.

But in Denmark? Not so much.

Lovibond and Palintest products are available in various European countries, but the cost for the capability is 3 - 4x what it is here, and availability is spotty.