Good afternoon everyone, I've stumbled upon a problem in my goldfish tank. Recently, my fish have started to gasp for air and it is causing me quiet the distress. I have a great pump, filteration system and have 3 fish in a 40 gallon aquarium (they're medium sized). I've done water exams and everything seems to be fine. I've noticed at one point my ammonia was a bit on the higher end of the scale so I added an ammonia reducing cartridge to my filter. I do constant 50% water changes and have never had a problem. The fish just look like they're in discomfort and I hate that I'd like to do whatever I can to help them feel better. Any advice?