Is your tank cycled?
What are your specific readings for ammonia, nitrate and nitrite? We cannot help you until you give us your readings. Also, your tank is overstocked with the common pleco. Plecos need a minimum of 75 gallons, as they grow 20 inches or bigger, if I remember correctly.
If your fish are gasping for oxygen, a CO2 bubbler will make things worse. CO2 is only for planted tanks with higher maintenance plants.