Recently my male guppy keep dying. I have lost four of them. What i notice is their tail become smaller and smaller, the tip of the tail finally become a point. The shape of their tail is like a triangle. And after a few days they died. This happened after i bought some yellow guppy and put them in the tank. But all the new yellow guppies are very good and healthy now. I dont know if it is related to them. Any body can help me out? Thank you.

Hi and welcome to TFF
Need alot more info first please;
a. How many gallons does the tank they are in hold?
b. How long has it been set up for and how long did you wait before adding fish and did you add them all at once to begin with?
c. What are your recent water quality stats like ammonia, nitrAtes and nitrItes?
d. How many fish do you have in total/their individual numbers and types/species?
e. Did the dead guppys show any other symtoms/strange behavior leading up to their deaths like heavy/rapid breathing/hanging around the bottom or top of the tank alot, or flicking/rubbing themselves against objects? do the injured fins/tails have any unusual texture/color to the edges of them?

I have had a similar problem my male guppies always die.
My tank is well established not over stocked and tank perameters are fine.
The males never show any signs of illness but after a fewweeks of having them I wake to find them dead.
The females never have any problems and there is no bullying in the tank.

Long tail fancy male guppys don't survive long in large tanks, particually the very pedigree ones as due to them being bred in small tanks for so many years they no longer have the muscle to survive in a large or strongly filtered tank; their tails just split or fall to peices over time...But i would check out other options first.

It's a 10 gal tank.

I have the tank of fish for about half year, and every thing doing well. it has two green cory, 2 catfish. the others are guppy.

I put the bag of the new fish in the tank for about half an hour and add some tank water to the bag, then I let the new fish into the tank, all the process about 40 minutes.

the water quality are good, after a few days of the adding new fish, i found two male's tail shrink together, it looks like the fish tail cant open up, it become smaller and smaller, the tail color is normal, no special texture or rubbing, They lost appetite and become thinner and thinner. then it died. Every fish died has the same shape of tail, it shrink together, the tail tip finally shrink together and become a point. and fish tail look like the shape of triangle.

Yes but what are your water stats? low stats don't nesarsarily mean healthy tank.
How many guppys do you have in total and what type/species exactly are your catfish?

ps: don't buy anymore fish until we know exactly whats wrong with the tank and/or fish...