Old age or bad tank?

Hello
Just looking for a few suggestions. We have a 10g tank that we started with one female beta (beta bertha) and 5 zebra danios last summer (I think in July-ish). Well, Bertha died a while ago after a long battle. One danio died a couple of weeks ago (looks like he was chasing one of the smaller ones and tried to fit through a spot that he was too chubby for and got stuck). Those deaths were, for lack of a better word, justified. But in the last two days, two more danios have died. The one was the smallest of the batch (skinnest) but was acting normal and would play with all the other ones. The other was one of the chunkies. There were three (well two left) that were big ones (a good 2 - 2.5 inches). Now there is one chunky and one that we call "no tail" (I think the chunkies ganged up on him one time and bit his tail - but he's doing just fine without it). My question is, are the fish "old"? They're probably just touching a year give or take. The water has been tested at three different places and everyone says that it's fine. We did just buy a new plant (fake) and added some more stones but cleaned them all well before we put them in. Any help or suggestions as to what to do?
Thanks!

Hi there RVT,
I cant tell you what is wrong with your danios,
but they are not old. Danios live about 3-5 years.
When you replaced the rocks did you remove the old ones
and put new ones in? If so, it could be a mini cycle your
tank is going through.

It's pretty hard to kill danios, they're often used as cycle fish.
How often do you do water changes? There are other toxins that can build up and stress fish besides the ones we test for (ammonia/nitrite/nitrate).
Do you remember the numbers they gave you when they tested your water?
What is your PH? Depending your water source, and how well it's buffered you could be having large PH swings which can stress fish. I know several people whose water comes out of the tap one number and then drops off severely, they have to add buffering themselves.
Did you remember to dechlorinate your water before a water change?

Other than that I don't have any suggestions, besides perhaps some kind of disease or parasites.

Hi
Thanks for the reply. We added rocks, we didn't take any out. I don't know what the values were, everyone just told me that all the values were fine, no one gave me any numbers. Maybe I'll take some more water and get them to write everything down for me.
I do water changes about once/week (give or take a couple days depending on my work schedule) and add Prime every time.
My husband wants new fish for the tank for I don't want to add anything if there's a tank problem.

Definitely ask for exact numbers on tests,
often LFS people will tell you low ammonia or nitrItes
are "fine", which they arent.
It would probably be best if you invested in a master test kit,
then you could test whenever you want.

I completely agree with sneaky.
A lot of run of the mill fish/pet stores or chains consider at or below .25 nitrites and ammonia safe, so when they say the numbers are "fine" it's not necessarily the case. Longterm exposure to any amount of ammonia or nitrites can drastically decrease the lifespan of a fish. Some also use test strips which just aren't accurate.
Also get the numbers on nitrAtes (though these usually have to be awfully high to start killing fish).
As far as PH, the numbers in the tank may be fine when tested, but you must get the water out of the tap tested also. If the water is not well buffered it can come out of the tap one number and then drop quickly, stressing fish.
Take some water out of the tap and either test it right then or bring it in to be tested right away and write down the numbers.
Take another bit of water that has sat either in a bucket or in the tank for a couple of days and have that tested. Most water drops some (mine drops from 7.4 to 7.2 and never falls further), but poorly buffered water can drop much more significantly. If there is a large difference between the aged water and the water straight out of the tap there are many ways to buffer your water and still keep fish safely.