Sick Male Silvertip Tetra

[Fina]

I have a tank with one male betta fish, and 7 silvertip tetras. I got them 10 days ago, and up until now, all of my fish have been happy and healthy. Yesterday, one of my tetras stopped eating. Today I noticed he had become opaque, and turned a dark brown colour, rather than his regular bright orange, he has a tear in his caudal fin, his pupils are constricted, and he had separated himself from the other tetras (this is what initially caught my attention as they are schooling fish) and was somewhat lethargic.
I have tested the tank water for numerous things, including ammonia, nitrates, CO2 levels, O2 levels, and pH, and everything is at a normal and safe level for the fish. I have also separated the sick fish from the others in case he is infectious, but I am not sure if this was the right thing to do.
There are no veterinary clinics in my area that help fish, and the petstore from which I got the fish was no help either, as they simply told me that if my fish dies, they will give me a new one, which I am sure you understand is not what I want.
Any help or advice would be much appreciated, as I am not sure how to help him at this point.

 

[PheonixKingZ]

Possibly pop-eye. Here is @Colin_T’s suggestion for treatment:

“Pop eye can be caused by a bacterial or fungal infection or an injury (scratch).

The easiest way to treat it is with big daily water changes and gravel cleaning the substrate.

Clean the filter if it hasn't been done in the last 2 weeks.

Wipe the inside of the tank with a clean fish sponge.

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You can add 1-2 heaped tablespoons of rock salt, sea salt, or swimming pool salt for every 20 litres of water. Keep the salt in there for 2 weeks.

When you do daily water changes, add salt to the new water before adding it to the tank so the salinity (salt level) remains constant.

It may be bloated. Try to reduce feeding as much as possible. Do the other fish seem ok?

Not that this is the highest of importance now, but bettas are not community fish and shouldn’t be kept as such.