I do have some Ich X, Seachem Paraguard and Seachem Metroplex on hand. Would any of those be useful on top of the water changes?
There might be a contaminant in the well water that is irritating the fish.
The only other reason a fish would produce excess mucous is external protozoan parasites like Costia, Chilodonella & Trichodina. These normally cause cream, white or grey patches over parts of the body, usually the top half of the body.
The fish in the picture looks like it is completely covered in mucous, including the fins and this is normally water quality or chemicals in the water.
You can try adding salt (2 heaped tablespoons per 20 litres (5 gallons) of water). Keep the salt in the tank for 2-4 weeks. If there's no improvement after a couple of days of salt, it is not helping and you can stop using it. The salt won't affect your fish or plants.
However, you should wipe the inside of the glass down, do a big water change, gravel clean the substrate, and clean the filter before treating the tank with anything, be it salt or chemical medications. Cleaning the tank removes a lot of gunk and this means the medication can work on treating the fish instead of being wasted on the gunk.
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Metroplex won't do anything for this.
Paraguard and ich x both contain Malachite Green, which will kill external protozoan parasites. However, Malachite Green is carcinogenic (causes cancer) and is pretty toxic. If you overdose the tank you will kill the fish so work out how much water is in the tank before treating.
You also need to clean the tank before treating and increase aeration/ surface turbulence whenever using salt or medications, because they both reduce the oxygen levels in the water.
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EDIT: Also this is a 125 gallon live planted system with sand. I don't think changing that much water that frequently and vacuuming the substrate like that is readily feasible.
If you are using well water and it's free of chlorine/ chloramine, then just drain the tank down half way and refill it. It shouldn't be an issue because you don't need to dechlorinate the new water. Just drain, gravel clean and refill.