What happened to my fish?

I have a 10 us gallon tank with 5 neon tetras and 4 Platys I used to have 6 tetras but one of them died but I don’t know what he died from. He schooled with the other tetras he ate and was perfectly fine except for he had these two discolored spots on him I read so many things but I didn’t know what he had because he only had the one symptom. Just in case this happens to one of my other I want to know how to treat it, here a re a couple photos of my fish.

How long has the tank been set up for?
How long have you had the fish for?
Have you added anything new to the tank in the last 2 weeks?

Try to check photos on your computer and make sure they are in focus before posting them on here. Clear images make it much easier to identify diseases.

The fish could have neon disease, which is a bacterial infection that causes the blue line to fade and go white. the sick fish usually die within 24 hours of the blue line fading. Neon disease spreads rapidly and can wipe out a tank of fish in a week.

If more fish develop the same symptoms during the next week, treat the tank with anti-biotics.

It could also be a physical injury that got infected with fungus.

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Do a 75% water change and gravel clean the substrate every day for the next week. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

The big daily water changes will dilute any nutrients and disease organisms in the water and hopefully help reduce the chance of other fish getting sick.

Monitor the fish over the next week and if any more of them get sick, post clear pictures asap and contact the local pet shop to see what they have for treating neon disease and other bacterial infections.