What Have I Done Wrong?

[AlanR]

I have a 200 litre Aquamanta aquarium which is quite heavily planted.  The aquarium was cycled and six gold white cloud mountain minnows were added from an existing tank in November.  
 
 
I added another six gold white mountain minnows about a week ago and all fish appeared healthy, happy and  frisky with a lot a displaying and mating behaviour.
 
Since adding the additional fish there was no change in water chemistry. 
 
 
Today I carried out my weekly water test using my API test kit  which gave the following results PH 7.5 Ammonia 0 Nitrite 0  Nitrate between 0 and 5ppm.
 
All fish were observed schooling together and chasing each other around the plants while I did the tests.
 
 
In preparation for the weekly water change I used the magnetic pad to clean algae off the front of the tank.
 
Opened the top of the tank and found three fish dead.
 
 I can't understand what has caused it.   I have used the cleaning pad many times and the fish have been fine. 
 
Any ideas?
 
Thanks Al

 

[james_fish]

Is there any possibility of a major fluctuation in temperature Alan? What temp you running the tank at? 
 
James

 

[AlanR]

James, 
 
The tank is at a steady 22 degrees and does not vary more than a degree or so between day and night.  I understand that the minnows like cool water and after cycling  the tank I left the heater in but turned it down to 20 degrees to ensure that the temperature did not drop too much overnight.

 

[l_l_l]

I'm thinking the fish that you found dead might have been doomed from the shop already.
 
Sometimes they go through a lot of stress during transportation and deaths within the first week or two are to be expected.
 
Are the dead fish from the last addition?

 

[AlanR]

They do look to be the smaller newer fish. I'm surprised that stress would be an issue as the new fish settled in with the existing fish overnight and doubling the number of the minnows seemed to make for a happier group and with the plants there is plenty of hiding space. Although the introduction of the new fish did trigger a lot of courtship and spawning.
 
I am keeping a close eye on the the remaining fish which seem to  be behaving normally.

 

[l_l_l]

I really don't know what else to say, the only fish that I had die on me were fish that were newly bought.. Fish that were either already sick, or that were already weak.
 
When did you purchase them?

 

[james_fish]

Could be anything then Alan, unfortunately unexpected and unexplainable deaths will always be a part of the hobby, like mentioned they could of already been diseased in some way from the shop before you even purchased them.
 
Fingers crossed for the rest of the group and the best of luck
 
James.

 

[Tongue_Flicker]

I'm thinking the fish that you found dead might have been doomed from the shop already.
 
Sometimes they go through a lot of stress during transportation and deaths within the first week or two are to be expected.
 
Are the dead fish from the last addition?

True that.

 

[rodley]

To concur, I bought 4 tiger fish recently, v careful to 'drip' them in and one died a week later, rest are fine. V sad, everything fine with tank & water so suspect a weak fish. Sigh, it happens.

 

[westonsupermanc]

I now always place my new fish directly from a net and never use the shop water after floating the fish of course. Had problems in the past so don't trust shop water anymore. Minnows can put up with cold or warm water so doubt that is the cause.