Fish Dying with Great Water Quality

[Skmage]

I have a freshwater fish tank that has been up and running with 6 fish in it for around 8 months now. I decided to add 4 more guppies to my tank and they all died the next morning. I have no idea why because my water quality is fine. The PH is a little high, but it shouldn't be dangerous and it hasn't affected the other fish. The only thing I have changed is around a 2 weeks ago I added a Amazon Sword plant to the tank and suddenly got a brown algae bloom after getting it. But as far as I am aware, brown algae isn't dangerous to fish right? Here is my setup and water quality numbers.

20 gallon
5 guppies (male)
1 bolivian ram

Ammonia = 0
PH = 7.6
Nitrite = 0
Nitrate = 6ppm
Phosphate = .19
Temperature = 80°F

 

[Aiken Drum]

Did you do anything to acclimate the new fish to your tank paramaters before adding them?

 

[Skmage]

Did you do anything to acclimate the new fish to your tank paramaters before adding them?

I turned off the light, floated the bag in the water for 20min, added my aquarium water 1/2 cup at a time every 5min and then netted them in eventually. Same as I did for my others.

 

[SaraB]

It may have been a batch that the store just got in. Yhe stress if being shipped there, going into their ranks and then being bought by you probably stressed them out too much.