Ich goldfish and pleco

I have three goldfish (1common, 2 fantail) and a bristlenose pleco in my 75 gallon tank and a few days ago I noticed tiny white spots all over the goldfish’s fins, although nothing on the pleco.. immediately I thought it was ich because it looked just like the description I’ve read on the internet. After doing some research, I discovered that the amount of salt recommended for treatment varies from a half teaspoon per gallon to 3 teaspoons per gallon. I ended up decoding to do 1 tsp per gallon to start with, but should I up the concentration..? The first thing I did to treat them was give them a 50% water change. Then I slowly raised the temp in their tank a few degrees a day to where it is now, 78 F. I added aquarium salt mixed in a 5 gallon bucket of their water as well, adding about 2 cups every 10 minutes for about three hours until I had to go to sleep, and adding the rest at a slightly faster pace the next morning. Since I began treatment, two days ago, the fish are still covered in white specks if not more than before, and red markings have begun to show on their fins as well. This morning I found them all laying on the bottom, while normally they are up and begging for food earlier than me. I have also noticed them itching themselves and swimming either slowly or erratically, and one has been resting at the bottom a lot.

Are these normal symptoms for goldfish for after ich has started to be treated? Should I raise the temperature more, add in more salt, or continue to do water changes...? After the 50% water change, the water parameters were zero ammonia and nitrites, and 40 ppm nitrates, which isn’t great unfortunately, so I can do more water changes if necessary, but it’s also physically challenging for me and time consuming, so I may not be able to do multiple large ones..
Any advice is appreciated, thank you very much!!

 

I use this when I do have a outbreak it works awsome

 

Thanks!! Do you have any idea if it’s works with salt, or if they two at the same time are two stressful for the fish?

 

Should be safe for both but I've only used it on my freshwater tanks

 

To update... I decided to move the goldfish and pleco to an empty 27 gallon. I moved about 10-13 gallons of the water they were in that had salt into it and filled the rest with clean water treated with prime. The temp is now at 82 F, and the tank is empty except for one piece of driftwood and one fake plant, both mostly for the pleco to hide near. I went to my lfs where I was recommended both API Super ICH Cure and E.M. Erythromycin to use together which I am currently doing. It has been two days since I started and the goldfish are only looking worse, while the pleco seems to remain unaffected..

 

I use this when I do have a outbreak it works awsome

Is this invert (snail) and pleco safe???