Two Sick Goldfish! Please help!

[masonrigsby]

Hello! I own two goldfish in a 10 gallon tank. I have had them for nearly a year without any issues. Yesterday, I noticed that one of the goldfish became very idle and sat at the bottom of the tank for most of the day. Today, it's irregular behavior continued and I noticed it was struggling to swim straight. I also performed a routine water change today, and after I changed the water I noticed my other goldfish started displaying similar symptoms as the first one. I have gotten busy and probably been cleaning the tank every other week rather than every single week, but the tank is clean now. I change out the filter, scrub the walls for allergy, clean the gravel, and change the water. I don't really have any ideas on how to help my goldfishes and right now they are both huddled together at the bottom corner of my aquarium barely moving at all. They are both having trouble swimming against even the power if the filter (meant for a 20 gallon tank), which they have never had trouble with before. Please let me know if there is anything that I can do to help my goldfishes.
Update: One goldfish is still moving, but floating on it's head on the bottom...

 

[Aiken Drum]

Did your fish get ill before or after all the cleaning?

Do you know your water parameters? pH, ammonia, nitrite, nitrate?

Might you have forgotten to use water conditioner with your water change?

Might something have got in the water? Detergent? Hand sanitiser?

Some info on goldfish.

Goldfish get big. Like 12 inches +. And they live a long time Upto 30 years. And they are very messy.

This all means they need space to grow and comfortably live. Much more space than a 10g tank can give them. When they dont have space to grow, their bodies stop growing, but their internal organs dont. This will cause ill health and early deaths. What you are seeing could be the end result of keeping goldfish in an unsuitable living environment.

 

[masonrigsby]

The first goldfish got sick before I cleaned the tank; while the second one didn't get sick until after.
The pH of the water is 6.2. I unfortunately don't know the ammonia level of the tank because I do not currently have any test strips for that, I only have pH ones. If ammonia is the issue is there any other way I can help the fish?
I used the proper amount of water conditioner in the water change and the temperature was about the same as the fishtank as well.
I don't think anything got in the water, as far as I know...
Thank you for the information on goldfish. I appreciate the knowledge and will definitely use it in the future.

 

[Aiken Drum]

I would try and change most of the water. Probably a couple of 25% water changes a few hours apart, followed by a 50% water change after another couple of hours. See if that helps.

First thing to do whenever something looks off is change water.

Things look to have got bad pretty quickly though.