what can cause fish to die suddenly?

[fisheyes]

my neighbour has has 3 mollies die suddenly in the last 2 days.one of them he even watched it swin to the top of the tank then down again and then it just keeled over and died.they showed no signs of being unwell before this.

it is a newish tank about 7 weeks old and the other fish in the tank seem to be fine.

 

[shadow]

What are your water parimeters?

 

[fisheyes]

its my neighbours tank i can't check his water till the morning,but he has a betta,3 other mollies,3 catfish and a couple of other fish in there.

 

[fisheyes]

OK another one of his mollies died today aswell are they particulry susceptible to anything?

 

[shadow]

Usually mollies are the first to show that something is not right in the tank. I would check the ammonia levels. What size is the tank?