Crash and Burn

[RFeldman]

So my LFS says I have just had a "tank crash" all but 3 fish... 1 Mollie and 2 blue gaurami are dead. 4 90% water changes later and my ammonia is .25ppm nitrites are 0ppm nitrates are 40 to 80 ppm and pH is 7.6 to 7.8. I think I am doing OK, just need to get my PH down a little more. How does that sound?

 

[Alyxx]

So my LFS says I have just had a "tank crash" all but 3 fish... 1 Mollie and 2 blue gaurami are dead. 4 90% water changes later and my ammonia is .25ppm nitrites are 0ppm nitrates are 40 to 80 ppm and pH is 7.6 to 7.8. I think I am doing OK, just need to get my PH down a little more. How does that sound?

Your ph is fine. A stable ph is much more important than having a specific number. Most fish are tank bred and are pretty resilient as far as the range of ph they can tolerate goes. What you need to bring down is your ammonia and your nitrates. If those numbers are after a 90% change, your ammonia and nitrate levels must have been sky high, which is probably what killed your fish. The dead bodies also caused the ammonia to rise once the fish started dying. Has this tank been cycled before you added fish? Did you add all the fish at once? What is your weekly maintenance schedule like?