I would raise temp to 86 or 87F (raising to 81 isn't going to do much).
I'm not sure what you mean by "added a forth dose of salt," but I've had good success treating Ich by getting temps up to 86F and maintaining 3 Tablespoons of salt per 5 gallons of water. I've use this method many times and have been able to save even sensitive fish, like clown loaches.
I also typically like to do daily large scale water changes (40 to 50%), vacuuming the substrate each time. This helps to remove Ich that aren't attached to your fish. Just be sure to maintain salinity of 3 Tablespoons per 5 gallons when doing the water changes.
Good luck!
Edit: wow, that poor knife is pretty well infested. Get the temps up quick! I had loaches that actually looked worse than your knife make full recoveries. There is hope!