Help - trouble with neons

[Spl00sh]

I bought 20 neons a few months ago, and they've all died off. They start losing colour, their tail fins shrink down to nothing sometimes and then they die. I have 6 left and they aren't looking good plus an old aged harlequin rasbora whose fins are looking a little off.
Water parameters are ammonia - 0 nitrite - 0 and pH is 7.4, tank has been well established for well over 5 years. All other fish are active and healthy, I have a new batch of fish to go in who are currently sitting in a QT tank. Can anyone help? These are the only symptoms I can see. I have included some pics of my last remaining tetras which I have placed into a breeder box TEMPORARILY only for tonight tonight keep them separate from the healthy.
I am in the UK

 

[Lynnzer]

It has to do with water quality as far as I can see.
How are you testing it, what kit do you use, and where do you get the water from?
Even if the water parameters are OK any infection that is present wouldn't show up on the tests.
For the good of those recently bought fish, don't mix them with any of the others that are affected, and don't put them in the same tank. In fact don't even use water from that tank or do simple things like use the same tools to clean both tanks. One tool for each tank, never put your hands in the affected tank then into the other.
I think that if it is fin rot then you are going to lose the infected fish so be prepared to do a thorough clean of the tank for a complete restart.
I know that not everyone will agree but I would euthanase them right now rather than watch them suffer any longer and die anyway.

 

[Biancah76]

I would agree that they are past saveable, where did you get them from, Pets at Home are good at bad quality/ defective neons, I have 9, 3 from PaH, and they are much paler than others, like a cross breed between neons and gold neons, you can see the difference, 1 of them developed a tumour on its mouth and died just after Christmas, my friend had also lost 2 of the pale batch before I bought tank from him.

 

[Colin_T]

Can we get some pictures of the tank and the other fish (all the fish).
What other fish are in the tank?
What are the tank dimensions (length x width x height)?

What sort of filter is on the tank?
How often and how do you clean the filter?
What is the nitrate level?

How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate new water before it's added to the tank?

How often do you feed the fish?
What do you feed them?

How big are the tetras when you buy them?

What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).

Don't add anything to the tank until this is sorted out otherwise you could lose the new fish too.