New Fish = Old Guppy's Death

[gaogier]

Hello
 
I am fast losing fish, so fast advise will be great.
 
I got some new fish online (discount fish), who are all alive and well, to top up our fish numbers as old fish naturally die off (maybe 1 per month), store said this was normal. 
 
Anywhoo, we topped off the fish tank with 15 female and 5 male guppies, which had an unknown number of female and male already in the tank with about 25 baby guppy fish (all guppy tank).
 
the old fish have nearly all but died off including the baby fish. Is there a reason for  this? Anything we can do to stop this from happening?
 
I know the old fish are dead due to the colours of the fish.

 

[RobRocksFishTank]

Please see this thread:
http/www.fishforums.net/index.php?/topic/422952-start-here-with-your-emergency/

 

[gaogier]

Sorry, its not my fish tank, but i shall try and fill it out.
 
Tank size: - unknown about 24inch x 12 inch by 18inch
pH: unknown
ammonia: unknown
nitrite: unknown
nitrate: unknown
kH: unknown
gH: unknown
tank temp: unknown, at a guess about 23"c

Fish Symptoms (include full description including lesion, color, location, fish behaviour): unknown

Volume and Frequency of water changes: unknown

Chemical Additives or Media in your tank: unknown

Tank inhabitants: Guppy

Recent additions to your tank (living or decoration): Guppies

Exposure to chemicals: none?

 

[RobRocksFishTank]

Is there a reason for  this? Anything we can do to stop this from happening?

 
Unknown.

 

[AmtotheBurr]

Were the new fish quarantined? It's possible they brought something with them that killed the other fish. I really have no clue. Also, was the water the new fish came in added to the aquarium or were the new fish netted out and put in the aquarium?
Rob, that was not very helpful.

 

[NickAu]

 
I got some new fish online (discount fish), who are all alive and well, to top up our fish numbers as old fish naturally die off (maybe 1 per month), store said this was normal.

In my opinion this is not normal at all.

 

[omega59]

Can you get your water tested, that would tell us a lot of information going on. If possible liquid test.