Id say stress as well, and you should never use salt and heat for anything.
Check out this video on the treatment of ICH when you get the chance:
https://www.youtube.com/watch?v=IopYeLq_ftc
Not to mention salt is not good despite what you have always been told. Also, the higher the temps, the less oxygen in the water, not to mention most of your tanks bad bacteria and parasites (IE Columnaris) thrive in higher temps.
A little info on salt in the aquarium:
The idea that salt is a tonic for fish when it is kept in fish tanks at moderate levels (~ 1 tablespoon per 5 gallons) needs to be taken with a "grain of salt". If your fish are the most commonly kept community fish such as tetras, corys, angels or even the rasboras and most anabantids, these are fish largely from soft, acid, low-TDS (total dissolved solids) waters. The average tap water in the U.S and Europe is at least moderately hard and alkaline and is certainly not improved by increasing the "salt" concentration ... that very thing in which our water already differs most from the natural waters of these fish. Certainly many of these fish adapt well to our local water conditions and I am an advocate of adapting non-breeding fish to local conditions. This is far better for both fish and keeper than constantly battling see-sawing water parameters. But, acclimation to your tap water is one thing .... making your water worse than it already is however, is quite another.
The prophylactic use of salt to avoid common pathogens and parasites of fish by keeping some level of salt in the tank is also fraught with difficulty. True, the use of salt, usually accompanied by increased temperature, is an effective treatment for one of the most common ectoparasite which beginning aquarists encounter ... Ichthyophthirius multifiliis, or Ich. However, only the free-swimming form of this parasite is treatable. Neither the fish-embedded growth phase, nor the encysted multiplication stage that rests on or in the substrate is readily attacked by salt or any other medication. Thus prophylactic treatment is useless as it makes little sense to treat a non-infected tank on a chronic basis. Further, any treatment for parasitic infestation is a "medication" and thus the use of salt differs little from the use of any one of the other available Ick medications available at any LFS. The biggest difference in my opinion though, is that of all the medications available, salt is probably the least effective by comparison. As far as the use of salt to prevent bacterial pathogens .. BUNK!
another fallacy is the addition of NaCl for livebearers. These fish as a group are native to estuarine environments where the waters are likely to be hard and alkaline at least and may even be brackish. The addition of sodium chloride (i.e., "salt") does absolutely nothing to match these species native waters since salt as comprises only a small portion of the total "salt" concentration -- the more significant elements are calcium, magnesium, potassium, boron and silicate salts.
It's important to note that the term "salt" isn't limited to sodium chloride. Calcium and Magnesium salts of carbonate, silicate, phosphate and borate are actually more important to the biology of brackish water fish. If your water is at least moderately hard (GH and KH 8-12) then the addition of some NaCl will do no harm. On the other hand if your water is soft and acid the addition of sodium salt will do little good. You need increased buffering capacity as much as the addition of minerals to the water and NaCl alone does absolutely nothing in this regard. You will need to add crushed coral or aragonite, both of which are primarily calcium carbonate salts, in addition to 'salt' as sodium chloride or better still, use marine salt mix which contains the whole range of cations and anions (positively and negatively charged ions, such as Na+, Ca ++; Cl-, CO3--) found in the sea. This will adequately buffer your water and more closely match the native waters of most live bearers in question.
In fact, Doc Wellfish salt is aquarium salt; and aquarium salt, kosher salt, rock salt, table salt are all simply sodium chloride -- period.
With that said, I would say your fish is not happy about the salt treatment, or has developed something else do to the treatment and temps.
