I've had a lot of issues with neon tetras over the years. To the point that I just don't even try to keep them anymore. If you want something that in my opinion is more hardy, cardinal tetras get a bit bigger but would look great in a tank your size. Ich is rough, but the API super ich cure has always worked great for me even without raising the temperature in the tank. Might be worth trying next time, although I hope that's the last time you see it for awhile.