Okay so I have a 20 gallon “beginner aquarium,” I added fish to it when I first got it and all but 3 died. Today ( a month later) I added more fish to it plus aquatic plants. I was told to add CO2 tablets for the plants. I did (2 tablets, 1 for every 10 gallons) and soon after the fish started bobbing on top of the water lever, not all but the majority are. I suspect this is because the CO2 caused an oxygen deprivation.. Either that or ammonia is high. I performed a 20% water change and also added biological aquarium supplement and a double dose of water conditioner in an effort to lower the pH or ammonia if it is high. The aquarium is about a month old so I do not think that the nitrogen cycle is a major factor. I’m hoping that the water change will increase the oxygen (if needed). Is there anything else I can do? I would appreciate any suggestions. Thanks guys! I’ll return the favor by answering your questions.