There's a bunch of different suggestions on how much that I found online, but I went with a happy medium which was 2 table spoons per 5g of water. I added half the tanks worth on e day then the other half 24hrs later so I started counting the days after the full amount of salt was added.
Without casting blame and only from a point of wanting to help, I have suspicions that you've never fully gotten rid of it to be honest. I've been battling it as of late, while previously I was like Andrew and I hadn't dealt with it in double-digit years.
Ich isn't something that always persists within a tank. It requires cycling through it's growth stages, which at some point requires a host. The length of the stages is dependent on temperature. I know that higher temperatures it cycles through stages faster, but that 10 day period IMO shouldn't start until you've observed no spots.
I'd also insure that for the last 5 days, no additional symptoms are observed. The ich "swarmers" that don't find a host die, but if treatment isn't kept up you might get some sticking around on the gills, or other hard to see areas that allow it to persist.
There is also a strain that isn't as easily impacted by salt and a strain that can withstand 90F, though supposedly they are independent strains.
I used Nox-ich to treat the new cats and loaches once I realized they still had it, but that wasn't before via water changing equipment managed to transfer it to my show tank after one Sunday when I decided to do my entire fish room, cats/loach quarantine last, then decided to do the show tank as well. Shit really hit the fan then.
After testing with a scrap piece of background, I found Nox-ich was going to stain my tank decor green and tearing it all out was going to be a HUGE PITA. Non-staining medication was going to run something like $150-$300, so I went with salt and heat too. For salt I'm using "Solar Salt", a naturally made NaCl advertised for water softeners ($6/40lbs instead of pricey food-grade salt or premium aquarium salt; keep in mind that I'm measuring in cups rather than tsp or tbs).
I'm running 1350 watts of heaters to reach 86F and started out with the same amount of salt per gallon, though I've been slowly upping it 48tsps at a time, now running a salinity of around 1.006 (the original salt amount started around 1.002). I found it took 5-6 days for the white spots to finally disappear from my big loaches, but am going to keep this up for another 1-2 weeks to insure it's eradicated.
Based on my past experiences with ich, I think the medication treatments with malachite green or copper (wouldn't recommend copper though) are a bit more definitive when it comes to killing that stuff. The salt/heat route doesn't have the same quick results all the time.
As for the rest of my tanks, I don't work on any of them on the same days as I deal with the 2 problem tanks and insure my equipment fully dries out for a period of a day to insure any parasites on it die off.
