fish question

Question
hi my name is jeeva. I have big problem in my tank. My tank is 25 gallon running on aqua clear filter with tetra such as black neons, bleeding hears and rasbora.
I have the tank running about 1.5 years. The tank maintance high quaity water. Everything was fine untill i bought 5 neon tetras that was on sale. Soon as i introduced in my tank i noticed one died couplr of daydays after. It is not ammonia proble, or anything with water. After couple of days my black neon had a infection in the tail. Later it died and after cuple of days another one died and also next day neon tetras died. I started reseaching and i found taht it  was a neon tetras disease and there notmuch cure for it. Alothough it only affecting black neon and neon tetras. Other rasbora and other tetras are fine. I changed 30% of the water still one died today. Right now all my black neon died only one left, i think he is going to died soon too. I had black neon for a quite long time. Only one had infection other black neon died without any symptom. They ate, they were active after couple of days they start suddely dieing one by one suddendly. I check the water quality everyting is fine.

What can i do?

Answer
Hi Jeeva,
Are you absolutely certain the water quality is healthy for your fish?
As in your levels read as -
Ammonia- 0
Nitrites- 0
Nitrates - 20 or less

Also, how often do you change the water? If you dont change it often enough, the Neon tetras (who would be use to cleaner water) may have gone into shock. Neons are reasonably hardy fish, but they can't stand drastic water chemistry changes.

I'm reluctant to think it may be neon tetra disease. While that is a good guess its usually something that develops overtime. And not so suddenly as in your case.  

Sometimes you get ahold of bad batches of neons too. They may have been a new shipment and their systems may have been weak before you even bought them. One good thing you can do for next time is ask the petstore how long they've had the neons. If its been at least a week everything should be just fine. Just make sure to acclimate them carefully as always.  Floating the bag for 15 minutes, then addding half a cup of your tank's water into the bag. Letting them sit for another 15 minutes and repeat until the bag's volume has been doubled. Usually this routine is sufficient enough to successfully acclimate the new fish with minor loss.  

I hope this helps!
Best of luck!
Susan~