Fantail Guppy Problem

[chewit]

I got a cube 17l with heater
Brought 3 male guppies all i have in the tank an 1 plant
An i have discovered that there is a bully he nipped half of 1 of the tails an he died
An got a fin of another who seems really sick
I phoned i shop an they say he wouldnt do that unless there sick.
Can anyone help

 

[WILDER]

How long has the tank been set up.
What are your water stats in ammonia, nitrite, nitrate, and ph.
Does the tank have a filter?

Even though guppys are small fish they still need a decent size tank of 10 gallons. They are active fish so need room to move.

Your tanks size is not that big for most fish.
It's only really fit for a few fish like a betta fish.

Immediate water change, increase aeration.

Bless Him.
R.I.P.