New fish keep dying

Question
I have had a small freshwater tank (5.5gal) for about 5 years now.  I usually keep 1 bata and 3 or 4 neons in it.  They have always co-existed quite well, lived out their lives then I would replace them as they died off. Over the last couple of months I first replaced my neons which had died of from old age, putting the new neons in with and old bata I had.  The new neons would all die off within a week.  I would replace them and the new ones would also die off with in a week.  So I gave up for a bit.  Then my very old bata died off so I got a new bata. The new bata is doing fine so I tried to get some more neons.  Once again they died off within a week.  So I tried a different pet store, but the same thing happend.  So I tried to put 3 platies in the tank.  Well they all died off within a week!  The bata is still doing just fine an dandy.  The water tests just fine as I use the testing strips.  I live in New Hampshire in the USA.  Could all of the pet stores be getting sick fish?  I find it hard to beleive there is something suddenly wrong with my tank as the bate seems happy enough.  I am taking care of my tank the same way I have for years.  Do you have any suggestions why this is happening?  I would really like to get more fish, but I don't dare. Please help.  Thanks

Answer
Hi There,

There could be several reasons:

1. Neons dont handle stress too well and it can be common to lose a few within a few days of adding them to the tank.
2. Disease, the most common for these being the neon tetra disease a tell tale sign of this is that they lose a lot of their color before they die.
3. They are being bullied or eaten by other non-compatible tank mates
4. Your water conditions are off. you need to check for level of ammonia, nitrite and nitrate mostly but PH can also a factor.

Hope this helps,
Chloe :)