Guppies Dying, Should I replace all the water for the new ones?

[Kamildomingo]

Just bought some guppies and I think they are sick as they are dying one by one. They have white strains of poop and is swimming sideways. I'm planning to buy new ones but my question is, should I replace all the stuff in my tank to avoid cross contamination?

 

[Aiken Drum]

Lets see if we can figure out whats wrong first. White poop is normally an internal infection, but can you read through the unhealthy fish sticky (link below) and answer all the questions as thoroughly as possible. If you dont know the answer to a question, say "I dont know". A photo of the fish and poop would help too.

https://www.aquariumadvice.com/forums/f17/before-posting-about-unhealthy-fish-read-this-32451.html