Poorly tetras

[SouthDownsSunshine]

I've got a 50L tank of mixed tetras and corys that was doing really well until weer introduced some neons.

Half (3) of the neons have died, one of my corys, and now my original red eyes are ailing. One disappeared overnight, and now another is looking bloated, breathing awkwardly and has a long strong of white coming out of its bum :(

Any ideas on what the problem might be and how I can help? Should I remove the poorly tetra before it dies?

Long white string of poo could be constipation, bacterial infection, internal parasites. Don't know what to advise really as I don't know what your water balance is. My experience of buying little fish is that I've sometimes had complete wipe outs as a result of buying a bad batch

I hope we don't get a complete wipe out :(

Is the tank cycled?
How long has it been set up?
How many of each kind of fish do you have? When you say "corys", what species?
What filter do you have?
How often do you change water and how much do you change?

Do large, frequent water changes to limit infection spreading to other fish. Always dechlorinate fresh water.

I'll try to help if you answer the above questions.

Thanks tree

Yes, tank was cycled before I added any fish, and I added them slowly. I've had it 3 months now.

I've got 6 red eye tetras, 6 smaller tetras (I can't remember which), 3 (was 6) neons, 2 (was 3) albino corys.

I've been extra careful with the tank. I change 25% a week, always dechlorinating. I test the water regularly as well and it seems fine - within healthy ranges. I add healthy bacteria each time I change the water. I wash the filter in the old water. We have a live plant that's thriving and being nibbled. We feed the fish tropical flakes.

I don't know what the filter is, it came with the tank.