Hello LittleWitch and Welcome

to Fish Lore! So sorry to hear that your fish are ill. Do you have a
test kit so that you can post all of your readings for ammonia,
nitrite and nitrate? If so what type of kit are you using? What is recommended most is the
API (liquid, not strips) Master Test Kit. The strips are very unreliable.
I would like to suggest that you rehouse your Plecos as they are tropical and your goldfish are cold water species. Although they may be surviving together, one or the other is not going to flourish and thrive as they should. One or the others live span may be severely shortened as well because of improper housing.
All of your fish in this tank are major waste producers. A 50% water change and adding some Prime water conditioner may help as well. I don't feel that you're changing out enough water during your water changes. The slightest amount of ammonia can be deadly to your fish. Your readings should be 0 ammonia, 0 nitrite and below 20 nitrates. Anything other than this means your tank is either not cycled or going through a mini cycle. If you're showing ammonia at all, do the water change daily, add the Prime to detox the ammonia (Amquel + and Ammolock will do the same) for the 24 hour period until it's time for the next water change. Do the water changes daily until you have the readings I mentioned above.
Is your quarantine tank cycled?
Best of luck for you and your fish. I'm sure you'll get additional responses soon as well.
Cleaning your filter: your filter media should only be rinsed in siphoned tank water once a month. Use it until it's falling apart and then you can replace a little of it at a time. You never want to use tap water on your filter media. The
chlorine can destroy all of the good bacteria that is needed to sustain your tank. If you use
activated carbon in your tank then it should be replaced completely every 3 weeks. It sounds like you may have done some "over" cleaning and your tank is in a mini cycle with high amounts of ammonia.
Ken

Little Witch I have merged your two threads since we are dealing with the same tank and issues. This will make it a lot less confusing for other members and to keep your responses all together.
Thanks!
Ken