Nitrite spike after water change

[trkchezz]

I did my water change yesterday like always, just at night and not in the morning. I didn't touch the filters (2) only replaced the filter floss. This morning I noticed my discus dark, not moving, breathing at the surface. I added some safe, as I didn't have time to address much else.

I went home for lunch, did a water test and Nitrites were about 1ppm, I did a 40% water change, then another doubled the does of safe and left.

What would of caused my cycle to crash?

75 gallon planted

 

[bazook]

Curious, how old is the tank? I have a 6 mo old tank that just finished its cycle.

Did you do 100% filter floss change?

 

[bazook]

Also, what kind of filtration?

 

[trkchezz]

2 years or so.

I run two filters, aquatop 500 and an AC70. I changed the floss in the AC and didn't even touch the Aquatop. Normal weekly routine