We usually do 20%water changes every week and gravel vac every 2 weeks. We did a 50%water change about 1 1/2 weeks ago when the Oscar started looking like this. Our tap water is slightly acidic and soft about 7.2 ph.
With big fish like the cichlids and pleco you have, you want to do bigger water changes and gravel clean the substrate every time you do a water change. These big fish produce a lot of waste and a 20% water change each week is not going to remove much gunk.
I would be doing a 75% water change every week and gravel cleaning the substrate every week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn.
Filters should be cleaned at least once a month otherwise the fish are more likely to develop health issues.
Add some salt to the tank with the sick fish and hope it survives.
--------------------
SALT
You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, Bettas & gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria, fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.