Arofanatics Fish Talk Forums - Death by... AIR POLLUTION?

Hi there fellow hobbyists, I am curious to know if anyone has experienced their fish dying due to extremely bad air condition.

For example, the haze in combination with the burning of the joss/incense paper's toxic smoke blowing directly into the house. I felt like I was in a gas chamber of some sort as I found it hard to inhale the the harmful smoke that permeated my house. Today was especially bad since its the last day of the hungry ghost festival? (Sorry, not too sure since I am more of an agnostic myself) I think the burning of the joss/incense paper went on for the entire day with people taking turns to do it.

I believe this is further worsened by one using an OHF (overhead filter), where the system coincidentally acts similar to a simple air purifier. The stackable trays allow the water to be in constant contact with the air which is a good thing normally, but when the air is so polluted with impurities, I am not so sure thats a good thing anymore.

Also, the OHF outlet where the water is gushing down seems to me like it is constantly forcing the toxic chemicals in the air to dissolve in the water.
The toxins that are dissolved into the water are I believe rather colourless and odorless, which makes it hard to even detect that something is causing harm to the fishes until its too late.

So today I fed my goldfishes at 3pm, both were perfectly fine, eating and swimming normally. I went out at 4.15pm, and came back around 5.45pm. I turned on the lights and noticed one of the goldfish swimming erratically and had trouble breathing. The other one was listless and seemed tired, which is not normal behaviour considering it was usually swimming up to the glass to "beg" for food.

I continued to observe the one with breathing difficulty for a few minutes and before I could even make sense of the situation or diagnose the problem, it had stopped breathing completely. The mouth no longer opened and closed. It could only drift in the current of the water.

I placed my hand inside the water to handle it, and determined that it had indeed passed away. I then wasted no time, filled up a clean bucket with water and added anti chlorine and placed the other still living fish into the bucket. After a few minutes of being listless, it became normal again.

I then drained the tank water and then refilled it whilst adding anti chlorine, hoping not to destroy the thriving colony of beneficial bacteria in the OHF. Thereafter, I added the fish back into the tank, and I am happy to say that the surviving fish is back to normal.

A close shave. If I had been back home an hour later, both could have been gone.

Just some background info - My routine for about a year has always been changing 75%-80% of the tank's water every 3-4 days. I have not done anything out of the ordinary as well. Just proposing a theory here that my fish's death might have to do with the extremely bad air condition.

Thank you for reading. I look forward to see my fellow hobbyists' inputs on the matter.