You have to remember smaller fish are far more sensitive than larger fish to water quality and changes.
Yes, the salt and increased DO is the cure for brown blood disease. Lowering your temp in theory would allow your water to hold more DO. Warmer the water the less it holds. It also slows down the metabolism of the fish, they will eat less, poop less. The salt does a lot of different things, all good ones for them. Not gonna get into all that. IF you intend to do 20-30% water changes, I wouldn't worry too much about salting again, unless meant as a preventative, but space these water changes out. The fish have to get used to the new concentrations every time. The idea is to dillute it back out, so the plants will like the water again. Salt will not evaporate, so as the water evaporates the water will slowly get saltier, you have a very very very low concentration. So not much of a concern but none the less a concept you need to understand if ever put in this position again. NO SUDDEN CHANGES. Which, if you are going to keep fish long term you will more than likely have this run in again at some point. But now you know how to fix it. The purpose of this was to buy your filter time to build up the bacteria. That's it. So, if you start doing small water changes, it will dillute out both the salt, and the nitrates and will ease the fish back into a more 'normal' for lack of better terms water. Salt in system is not a bad thing though, as long as plants can tolerate it. Some folks keep it in there all the time. I don't as my strawberries don't like it....at all. But my wintering over tanks get salted. Keep the aeration up, and the temp down and really light feedings...the more you feed, the more they mess, the higher the pollution in the system which you are trying to compensate for now. It's a cycle. A big circle called the nitrogen cycle, you have to allow the bacteria to build, and break down the pollutants that the fish, fish waste, and what not build up. Maybe overally simplified here. But that's the basics. Studies have proven though, and yes I can find the PDF on this too if anyone needs it, that prolonged exposure to brown blood disease has negative effects on fertility. So the faster you get it fixed, and the faster you understand the WHY's behind it the better off your goona be.
I only wish I was a fish Jesus. I know what I know, cuz I have killed thousands of fish in making mistakes, and in testing out the WHY's, How Come's, and What does this do? Ideas that I have had. There's lots of data available online, but it only goes as far as someone has documented. I am not a biologist, but I have volumes of marine biology and fisheries texts. Some is dated, and I don't know all the scientific processes and terms. But I have learned you learn far more by actually doing it, putting the hands on, and losing a few....than from the book. Remember too, just cus someone tells you something, or you read it online doesn't mean it's true or real. ALWAYS do more research, google, etc to verify. If you see the same answer in enough places then you start having a direction. I am not saying anything bad about anyone, or anything like that. Just saying it's your money, your fish....always verify. That's why I told you it was brown blood disease upfront, so that you could google the treatments for it. There's a lot of data out on this.