Help- zebra danios keep dying after addition of new fish

[Tinyrioters]

I had 4 danios in a 10 gallon tank, the tank has been cycled for 3 months. I recently added 2 new danios (2 weeks ago). I didn't quarantine them as they came from the same tank I had bought my old ones from. Over the past two weeks the new 2 fish have died. They would spend all their time isolating and just swimming against the filter before eventually dying. I figured it was just stress but now one of my older fish is displaying the same symptoms as the others. My ammonia and nitrite levels are 0, nitrates are 5, and pH is 6.8. The temperature is 73 F. Should I do a water change? Thanks for any advice.

 

[Aiken Drum]

First thing you should do if anything is wrong is change some water.

Take a read through the "unhealthy fish" sticky and give as much detail as you are able. It may bring something up that explains whats happening.

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