It probably had an internal bacterial infection. there's no cure for this.
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Wipe the inside of the glass with a clean fish sponge.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use them.
Do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Don't get any more fish for a month.
This will reduce the risk of more fish getting sick
------------------
Wipe the inside of the glass with a clean fish sponge.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use them.
Do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Don't get any more fish for a month.
This will reduce the risk of more fish getting sick