OK, just to clarify something: your probably fish didn't die from the whitespot. Whitespot rarely kills fish quickly. What it does is irritate the gill membranes (among other things) and this, over time, can lead to suffocation and death. However, to go from infection to death takes weeks probably, and if you treat the tank, there's no chance of that happening. Whitespot is perhaps best thought of as the pneumonia for fish, potentially serious, but if treated, no big deal.
Right. With that out the way, assuming your fish did not have whitespot for weeks (and didn't look like they had been dipped in sugar) the question is what killed the fish.
My guess would be the immature tank. Cycling a tank is identical with having a mature aquarium. There's something difficult to describe about established tanks, but basically they have a certain inertia that prevents problems. Most fish fatalities are in tanks that are a few weeks old. Hence, even in a cycled tank, I'd recommend only using hardy fish to begin with. Livebearers are good, so are the cheaper Corydoras, kuhli loaches, rainbowfish, and a few others. Neons aren't hardy really. They may have been once, but...
Neon Tetra Disease! This is an incurable free gift with virtually every cheap (i.e., not wild caught) neon sold. It will infect a few other fish, but it is best known in neons and cardinals. Basically one gets sick, dies, and then a few days later another, and so on until they're all gone. Symptoms include loss of colour, swimming away from the school, hiding a lot, and not eating.
NTD is transmitted when one fish nibbles the corpse of another. It is ESSENTIAL to remove (and destroy) sick fish at once. It cannot reliably be cured, so at the very least, remove the sick fish to a quarantine tank and treat with Octozin there if you want. Removing sick fish is the ONLY way to break the cycle and stop the disease.
So, between NTD and an immature tank, I think neons are a bad choice. Stick with other hardy fish for at least a few more weeks, and then try neons again. Actually, save your money and buy cardinals. They seem to be hardier. Of course only buy from a shop that has a good selection of healthy fish, and carries out basic quarantine and health measures (e.g., removing dead fish, sterilising nets between use). Chain pet stores are usually rubbish in this regard.
Finally, check your water pH and hardness. Neons and cardinals are never easy to keep in hard, alkaline water and are invariable short lived. They should last 4 years. If you have hard water, stick to livebearers, rainbowfish, and hardwater killifish. Your life will be so much easier.
Cheers,
Neale
PS. Turning the heat up speeds the whitespot parasite life cycle, and so gets the cure done faster. In theory at least, but to be honest in tropical aquaria the difference between 24 C and 28 C is not that great. It does make a big difference with coldwater fish though.
