Abnormal behavior of goldfish

[Paolo997]

Hi everyone,
I'm Paolo from italy and every english mistake is the translator's fault combined with my poor english
I have a big problem with my female goldfish.
From january of this year, about every minute (yeah, every single minute of a day!) she swims to the surface, take a big bubble of air into her mouth, munch it at middle high, and then lies down with the belly on the bottom of the tank.
she also expels long, white and filamentous stools (sometimes containing little bubbles) like in the picture
at least she is hungry like a dragon as always...
my tank is a 600liters outside of the house in a shady place, in winter the temperature is about 5°C and summer 26°C.
1 sump of 100liters with biological filter
1 big recirculation pump
1 uv light for defeate unicellular algae
1 extra pump for oxygen
the water values are normal as always tested by my trusted fish store (they can't explain it )
she also lives with about 7-8 other goldfish, she is 8, i bought her when she was about 1 or less. Some of my goldfish are 10+ years old and everyone in the tank is good except for her
Can you help me please? I hoped that with higher spring temperature everything would be good, but unfortunately it is not happening
https://ibb.co/MgqRWYT
https://ibb.co/sqJD1ZZ
Thanks in advance
Paolo

NO3 25-50
NO2 0-0.5
gh 14-21 (has always been high in my zone)
kh 10-15
ph 7.2
cl 0
unfortunately i have now only strips value which shows a bit higher values from shop reagents

 

[Autumnsky]

So happy to have you join us! Welcome.

Fish poop with bubbles is a sign of constipation from all that I have read. Fast for a few days, feed peas for a few days, see how it goes then.

Do you feed the goldfish floating food or sinking food? She could be gulping too much air as well.

Usually clear poop seems to indicate parasites (which of course it can mean). But in Goldfish they have a problem of gulping in too much air and digestive issues.

Some fish foods now have probiotics in them and also pea protein. Not sure if that also includes the pea fiber. Adding in more vegetables might help as well.