There are several possible causes, and all together can very well kill quickly.
1) As mentioned, a sudden temperature change is very stressful for fish.
2) These are tropical fish, and as such require a heated aquarium. If not kept heated they will be well outside their comfort zone and be stressed. Stress = lower immune system.
3) Sudden water quality changes is extremely stressful. 1 gallon is an extremely small volume for these fish, so water parameters can swing quite rapidly (namely Ammonia, but also pH).
4) Use of 100% RO/DI water. This has zero buffering capacity for pH. You can get a catastrophic drop in pH simply from it absorbing CO2 from the air, which is acidic. Normally the KH in water will buffer it, but RO/DI water has 0 GH and KH.
5) The process of being 'cupped' is stressful for fish.
6) Salt can be used as a medication, but these are not brackish or saltwater fish, they are freshwater, and should not be kept long term in salt.
7) Not de-chlorinating the new water, you mentioned treating it though.
Stress has a major impact on fish, and as you can see there are lots of possible causes of stress.