neon tetras dying... any ideas

Hi,

I have had to resort to desparation to try and work out why my neons are
dying...

In the past before moving them to a new tank, I have successfully had a
school of around 12 neon tetras for about 9 months. I had moved them to the
new tank around 4 weeks ago and they seemed fine. I then have added another
12 to the school, to find that most of them have died off, and adding
another 18 didn't help as I am now down to around 10.

Have I introduced a disease to the tank??? The fish seem to be fine, and
then the following morning they are found at the bottom of the tank.

The two Ottos and SAE seem to be fine.

PH is at 7, Ammonia 0, Nitrites 0, Nitrates 0.5; well planted tank. I am at
a loss as to why and what I am doing wrong, someone suggested the tank size
is too big for them (20g) and they are dying of shock ?!?

Any comments, web references or anything else would be appreciated, as I am
losing them quite rapidly (over the last week or so) and at one stage 8 at a
time...

Des.