Hi, Jason, welcome to the forum!
In my experience, fish will not die quickly (within 24~48 hours) if it is a filtration problem. They usually last for days to weeks or even longer.
Your situation sounds more like the fish were in shock. Did it happen right after a water change?
I personally had made the mistake of letting the fish get into shock after a large water change. The problem was the temperature of the new water. It was about 6F below the tank water. That was enough to get the tropical fish in shock. Basically, any kind of sudden change in the water can get them into shock. Things like water temperature is just one of them. Other things you need to pay attention to is the water PH, hardness, and even nitrate concentration. Sudden large change in water perimeters will kill the fish very quickly. You can see the sign right away after the water change.
After my own incident which killed off more than half of my fish (many years ago), I started to use an extra aquarium heater to make sure the new water in the water bucket has similar temperature to the tank water.
Another thing is, how much water did you change in term of %? Usually 30~50% partial water change every week is recommended. Too little won't remove much nitrate and other organic waste. Too large of a water change will contribute to the possible "shock" effect due to the buildup of nitrate and the change of PH as well as water hardness. Water change every two weeks will also have more nitrate buildup along with PH and water hardness change. Basically, I recommend 30~50% partial water change once a week over your current routine. It will not only optimize your water quality, but it will also minimize the difference between the new water and the tank water.
If there is a problem with the filtration, you should check your water again for ammonia, nitrite readings. These two readings must be 0ppm or there is a serious problem. Nitrate is ok at even 40ppm+ for most fish, since it is not toxic like the other two chemicals.
You do not need a canister filter if your ammonia and nitrite readings are at 0ppm all the time.
I also recommend minimize the feeding. Too much food will pollute the water to cause ammonia and nitrite spike.
Please let me know if you still have questions.
