Fish Gasping For Air

[NormaJean]

I woke up this morning and found my L200 pleco dead at the bottom of my aquarium, shortly after I noticed all my fish at the top of the tank gasping for air. The measurements of the tank are 48"x24"x24", I've had the tank up and running for 3.5 months without a problem. I realize because of the tanks hight less oxygen would get in which is why the tank is heavily planted. I checked the ammonia, nitrite and nitrate it reads; Ammonia: 0ppm, Nitrite: 0ppm, Nitrate: 5ppm. So everything there is fine. I did put an air pump in there to help with the circulation, but my question is, why would everything be fine all this time, and now out of the blue I have this problem? Last night I did do a water change and trimmed the plants and took off some dead leaves, but that's about it. And I very much doubt that would be a problem since I do that every week. My fish are still at the top gasping for air, I can't figure out what's wrong. Any help would be greatly appreciated. The test kit I'm using is a Master test kit by Nutrafin, I also have another one to see if the tests are accurate and that one's by API. I got the same results with both,

 

[Hoveringpie]

Do you want inject co2 try putting a filter outlet to create maximum disturbance on the water to oxygenate the water faster, have you been working with chemicals that could have come off your hands into the water?

 

[NormaJean]

Do you want inject co2 try putting a filter outlet to create maximum disturbance on the water to oxygenate the water faster, have you been working with chemicals that could have come off your hands into the water?

I always wash my hands and arms I also put my hair up, I'm very careful with what goes into my aquariums. I don't use co2, just high lighting and fertilizers. I have a canister filter it's as high as possible. The air pump seems to be doing its job now. It's just weird that the plants would be oxygenating the water and out of nowhere everything just craps out.

 

[JohnDyer]

If you've done nothing different for months, it's probably some chemical. Do you use air freshener or cleaning spray type things near the aquarium? They cut off the fish's ability to get oxygen. Maybe there was something on the scissors you used to trim the plants?
If they still aren't back to normal, do a big water change, and add extra dechlorinator, that might clear it up.

 

[NormaJean]

The fish are doing better now. I don't use air freshener, I use scented candles. But unless someone poured it in the aquarium it couldn't have done everything. I use water to clean the tank, and no harmful chemicals have been used near the aquarium. The scissors are specially for trimming the plants, I keep them in a plastic bag under my aquarium.
Despite nothing being wrong I still did a 30% water change just in case.