Sailfin molly fry swim in circle

I have an outdoor 300 litters tank with about 50 guppies, sailfin mollies, and platy gold fries. I noticed they are flashing/rubbing on leaves, sand and rocks about a week ago. I though it is some kind of invisible fungus developing on their bodies and makes them itchy so I put in some salt and do 20% water change everyday for several days now.

It seems like they are doing much better but this morning I saw one molly fry was swimming while his body rotating in circle (like a drill) on the surface. I was about to bring him out to inspect but then he disappeared. I think one of the adults ate him.

Is this some kind of fluke? Should I worry about this? My water is 26 degrees Celcious and the pH is 7.5

Hi,
Without seeing them its hard to say but a good guess would be velvet aka oodinium. Its a common pathogen in tropical fish tanks and can get out control fast in heavy stock tanks with little water changes. You mentioned you treated with salt? How much did you use?

hth,
al

Hi,
I use 100 grams of salt for 100 litters of water once a week while doing 20% water change every day. Is it okay?

That salt concentration probably won't cure a pathogen like this....but more may harm the guppies.. I think I would look into something like Paraguard by seachem.

al