You can keep the lights off during the treatment which will help reduce stress but better to nip rot in the bud now than watch it deteriorate his body! Oh and you can also just get regular Rock salt instead of AQ salt as long as it is PURE NaCl otherwise just getting AQ salt at walmart or your LFS is the best way to go for this. It doesn't look too bad but he's certainly lost a lot of fins already and you don't want it to spread more
But daily water changes, add in the salt again because it doesn't evaporate and just make sure to acclimate him each time you put him back into his tank. And always dissolve the salt!!!!! <---biggest thing right there because it can hurt him if it isn't dissolved. But once it's dissolved it's fine as long as you acclimate him to his new water each day!
I use their old cups or plastic ware (must be new, never seen soap) and scoop them up and keep them in that while I change the water and then when I'm ready, float them in the new water and add about 1-2 tablespoons of new water to his floating cup for about 30-45 minutes to acclimate him well.
Also a trick to get the temp as close to 80 as possible or 78 if that's what your tank runs at with the non-adjustable heater, is to run a thermometer under the tap when you get new water. Watch the thermometer and adjust the temp to get it close enough to the temp he needs. I suggest you get a glass thermometer from any pet store or walmart, it's only a dollar something but you need to know the temp of your tank always to see if your heater is working properly
I hope this helps and any questions feel free to ask!