Fish All Dying

[peter_uk]

Hi there all,
My tank size is 35UK (42US) gallons I have had the tank running now for at least 3 months and I have got
1 dwarf gourami
2 platies
5 pandas
6 x-ray tetras
I normally use the gravel vac once a week and replace the water then.
All water tests that I can do are reading fine, the same as they always have
Ph 6.4, Ammonia 0, nitrite 0, nitrate 10.

The other day had noticed that one of my pandas was acting weird to say the least it would swim up to the top stay in a vertical position until it was upside-down then float down to the bottom upside-down and just stay there for 5 seconds before righting itself then later on it would do the same.
Next morning the panda was dead.
Today I come home from work to find out that all the tetras and pandas are dead. I have been told that all the other pandas done the same thing as above
The remaining fish now just seem to be motionless just doing enough to stay at the very top. I have done a water change of around 50% to hopefully give the rest a possible chance of survival.
None of the dead or alive fish seem to have any kind of visible defect or anything to would be the cause of the problem. anyone have any idea what could be the cause of all of these deaths.
thanks
peter

 

[scubadoo]

Your pH seems a little low, a bit too acidic, but if this has been stable for some time then its not likely to be the problem.

This sounds like it could be a case of poisoning - has somebody been using any chemicals in or near to the tank in the last day or so ?

 

[Miss Wiggle]

hmmm sounds odd for health to deteriorate so suddenly. could be something like poisoning.

panda's are notoriously sensitive though, they're delicate little things, the slightest change could kill them

 

[peter_uk]

one platy now died, other one not looking to good.
its swimming with its head up and tail fin down.

 

[black angel]

Is your ph always that low? If so then how long has the tank been running? Anything been sprayed near the tank and does it have an airpump?
How often do you do water changes and how much each time. Are you using a gravel vac?

 

[x-fish]

i think its to do eith the ph try get it to 6.9 or 7.