I have a well established tank (11-12 years) at home, and can successfully keep different types of fish. At the moment, I have the following
Tetras (cardinal and neon and rummy nose)
Corydoras - two
Sucking loach - one.
All live very happily together but every time I try to add male guppies, they last approximately one week, then the tails shrivel, the bodies arch and within 2 weeks, they are all dead.
I have had water tested and confirmed by retailer that all is OK in my tank, but just cannot keep these beautiful fish. I have told the retailer what fish I have in my tank, and they have advised me (on several occasions) that guppies should be fine with my current fish.
What am I doing wrong. Does anyone else have these problems. How come, they all seem so healthy in the fish shop?
Sorry to be so basic, but I am not a professional, just a novice really.

Tetras (cardinal and neon and rummy nose)
Corydoras - two
Sucking loach - one.
All live very happily together but every time I try to add male guppies, they last approximately one week, then the tails shrivel, the bodies arch and within 2 weeks, they are all dead.
I have had water tested and confirmed by retailer that all is OK in my tank, but just cannot keep these beautiful fish. I have told the retailer what fish I have in my tank, and they have advised me (on several occasions) that guppies should be fine with my current fish.
What am I doing wrong. Does anyone else have these problems. How come, they all seem so healthy in the fish shop?
Sorry to be so basic, but I am not a professional, just a novice really.