"Li khor boh wa khor" ( your suffering is none as compared to my suffering ).

Another fish died of the most unusual and mysterious cause.
Many of the fish was dark looking especially the 4 new fish I bought 2 weeks ago ( without quaranteen in my place ).
But they were the most eager eater among the fish and was gaining weight and size.
I did a PP treatment for both of my tank last Sunday.
Procedure:
Feed fish in the morning as usual.
2 hours later, 50% water change, add PP to 5 ppm. ( by mistake ) for 4 hr bath.
2 air line provided in each tank.
After 1.5 hour, all fish normal, add another 2 ppm for good measure ( and this is real good measure ).
Nothing abnormal after 15 minutes and I left home to attend to something else wiht plan to come back after they get the full 4 hr. bath.
Return half an hour later ( total time of bath 3 hrs ) to find that most fish hanging near top of water line gasping for air.
100% water change immediately.
Luckily no fish died, but some fish took more than half and hour to go back to normal breathing.
After that all fish appear normal although a little scared ( as usual after water change ).
6 hours later feed 2nd feeding of the day.
All fish as normal, appitite normal.
One hour later, water change.
During water change ( discharging of water ) I notice a fish swimming abnormally.
It was gliding across the tank like flying saucer and and appear to be struggling and then gasping for air.
As other fish begin to bully it, I decided to put it into a pail together with another fish under treatment for while feaces.
One air line was provided and after a few moment of struggling and gasping for air, it start to settle down at the bottom ( lying flat ).
On and off, it will stuggle to the surface like in pain and after a few moment, fall to the bottom again.
This continued for about 2 hours.
I start to add new water and adjust ph gradually to 6.0 over a period of 1 hour.
I also redose the water with acryflavin ( about 10 drops for 20 litre ) and add salt ( about 1 gram per litre ).
I notice that there is some swelling ( very slight ) at the stomach and decided to add Epsom salt at half tea spoon for 20 litre.
After all this, I notice the fish staying at a fixed spot at the bottom ( erect now ) of the pail for a long time.
I watch it for about 20 minutes and suddenly, it move away and start to struggle for air again.
I notice a patch ( about 10 cent coin ) of dark red things at the spot it used to rest.
When dispersed, I found that it was undigested food.
Red worm from Saturday night's feeding and beef heart from either Sunday morning or evening's feeding.
I suspect other fishes in the main tank may succome to similar problem and added Epsom salt to the other 2 tank as well.
Nothing happen to all the other fish.
But this one died the next morning without further "Lau Sai".
I notice that its stomach was also slightly bloated.

Any suggestion of what went wrong?

Also, one and 2 days after this, fishes in one of the tank ( where the dead fish come from ) have very strange behavior.

At the slightest sound, all fish jolted and jump about ( never happen before ).
Sometime even slight movement gets to dart helter scalter.
Checked, ph, has falled to 5.3 ( normally maintained at 6.3 to 6.8 ) before water change.
I was surprised by this ph drop as I change water 50% ( municipal water ph 7.2 ) every day and add phosphoric acid to adjust to 6.5 after water change if necessary.
I have stop the heater before and after the treatment and due to rainy season, water seems very cold ( sorry no reliable themometer yet ).

Sorry to be so long winded.

Your opinion and suggestion highly appreciated.

Cheers!

BT Lee