I can't see anything on the pleco. If you let the fish settle down in the tank and take a number of photos, you can check them on your computer and post a couple that clearly show the issue. When fish are lifted out of water their fins squish together and their colour changes a bit making identification of health issues hard.
Salt does not cause fin rot. Salt can damage the kidneys of some fish if they come from soft water and are exposed to high levels of salt for long periods of time.
Salt can be used to treat minor bacterial and fungal infections, as well as some protozoan infections. 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.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 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, 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 but the higher dose rate will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.
------------------------
Melafix & Pimafix don't treat finrot. They don't do much at all except make the house smell like a gum tree after it's rained.
------------------------
If your fish have whitespot for 3 months, you are not treating it properly.
Do the fish actually have small white dots on their body and fins?
If not, then what are the symptoms?
------------------------
Intestinal worms and gill flukes will cause fish to lose weight over time. Livebearers are renown for having these parasites. Infected fish will still eat. If the fish has lost weight rapidly (over a week or two) and is not eating, then it has an internal protozoan infection.
You could look for something like API General Cure if the fish has worms and an internal protozoan infection. You treat the whole tank. This product will kill filter bacteria so you need to monitor ammonia and nitrite levels after using it.
If the fish is still eating then it is probably just intestinal worms and gill flukes. Praziquantel on its own will treat tapeworm and gill flukes. You can also look into Levamisole to treat thread/ round worms. You use these medications separately. You treat all your aquariums at the same time. I would use Praziquantel first and treat once a week for 3 weeks. Do a 75% water change and complete gravel clean 24-48 hours after treatment.
You can use these medications with salt in the water.
------------------------
To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will absorb the medication and stop it working. Carbon does not remove salt.
Wipe the inside of the glass down, do a 75% water change and complete gravel clean. And clean the filter before treating. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase surface turbulence/ aeration when using medications or salt because they reduce the dissolved oxygen in the water.