[angiolo]

Sul forum di un sito inglese (kokosgoldfish) suggeriscono che può dipendere dalla patina oleosa in superficie provocata dal cibo e pertanto sarebbe meglio avere uno skimmer che la elimini:

The fish is slurping the tasty, protein-rich film on the surface of the water and gets air with it. This film originates from the fish food, so he does it after feeding.

Gel foods and soft or wet treats create the most surface film, followed by soaked sinking pellets, then dry sinking pellets. I've never heard of this problem with floating pellets. A tall tank has a lower surface area to volume ratio, and thus collects a thicker film.

You can remove the surface film with a skimmer. You can DIY or purchase one. Just make sure any skimmer you get specifies that it works in fresh water.

Francamente non so se è una risposta valida; si dovrebbe provare se le cose cambiano con un piccolo skimmer (ad es. Eheim skim 350).