Fish Dying with Great Water Quality


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


how are the other fish doing? was it a long journey for them from the place you got them? If it’s only the new ones that have died I would suspect them to have had a problem with acclimating to the tank. I doubt a disease would have killed them all simultaneously.

have you got enough disturbance at the water surface? Only other thing I can think is that when the fish are stressed they breathe more heavily and a lack/lower oxygen level could have created a problem? Just brainstorming as I’m no expert!

maybe someone else will be able to help more?


It sounds like an acclimation problem. Be sure to float new fish and add a cup of your tank water to the bag every 10 min until the bag is about full before netting the new friends out into their new home. Dump the remaining bag water in your sink. Pet store fish have been through the wringer before getting to their permanent home. From the breeder to the distributor, to the pet store, to your's super stressful.


When and how much do you do water changes? Next time test the water in the bag from the store. See how close or not close it is.
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