Um.... Yeah

[RFeldman]

Major fish kill when I got back from work today. I added 4 mollies, 1 Pleco, 5 glofish... 1 neon tetra 4 glass neons to my 75 gal that was home to 9 other small fish. Tonight, my 4 glass neon, 1 pleco, 2 angels and 2 loaches are dead. Did a water test, everything was fine. So I did an emergency 50% water change. Praying nothing else dies. What could be the cause?

 

[RFeldman]

The only thing I changed is I used melafix when I added the new fish, recommended by the LFS I purchased from

 

[armymp327]

seems you added too many fish at one time causing the BB to become overloaded, thefore allowing ammonia or nitrites to rise

can you post your test results for ammonia, nitrites, and nitrates. Also what do you use to test your water?

 

[RFeldman]

BB? My nitrates were 0 nitrites between 0 and 20. I use API test strips

 

[armymp327]

BB is beneficial bacteria do you have a way to test for ammonia?

test strips are very inaccurate, i would suggest picking the the API freshwater master test kit, it is a liquid test kit that has ammonia, nitrite, nitrate, low ph, high ph testing kits.

are you sure the nitrites are 0-20 and not nitrates?

im not sure if you understand the nitrogen cycle give this a read

The Aquarium Nitrogen Cycle

also this

Fish-in Cycling: Step over into the dark side - Aquarium Advice

i would also suggest doing another 50% water change until you are able to get the test kit i recommend and can test for ammonia, nitrites, and nitrates accurately