I have seen this before and then each started to get sick and some even died: first their dorsal fin fold down as they swim about. Then they find a place to rest more often. Then spots and other symptoms appear.

This has happened two times already.

I also notice it happened when I changed the water, but I had changed the water many times before so I'm not sure why twice now that caused them to get sick, especially the comets.

Did anyone else notice that early symptom with the dorsal fin fold down as they swim about and then get sick a day or two after that?

The last two times they got fin rots and white spots and I was able to save 3 and lost 4.

This will be the third time. Just today I notice the comets are doing that again, swimming with the dorsal fin closed down. I think it's an early sign they are going to get sick again.

Please help if you know what I can do early to fix it this time or what I can do to prevent it next time.
Thank you.

Thank you for your reply.

I sat the treated water for about 2 hours for temperature to be the same. Water in tank was fine, pH-wise. I changed it about 10 gallons each weekend. I have 5 medium size goldfishes in a 55 tank with the plenty of air bubbles and heavy duty canister filter system. I cleaned the filter system once a month as directed too.

The dorsal fin wasn't exactly clamped I wouldn't say, just closed down as they swim. Then they open it up just fine at times but they closed it down a long period of time more than usual when right after that behavior I noticed them get sick. I hope not this time.

There is no sign of rot or white spots on their fins or body yet.

Try to get a good look at their fins.  You're looking for red streaks, fungus, or parasites.  Hopefully it's just a water quality issue, but it doesn't hurt to be careful.

You could try adding some aquarium salt for freshwater tanks to your aquarium.  The salt will give the fish some electrolyes.  Also, Melafix is excellent by API. you can dose with that for 3 days in a row.