Hi folks, I was wondering if someone could take an educated guess as to what this bump might be on my peppered cory. Please let me know if and how I should treat him for this bump. Thanks for your help! Back ground information base on a fish tank questionnaire: What size is your tank? 20 gallon What temperature is your tank? Around 80 Does your tank have a filter? Yes. : AquaClear 50 Power Filter Does your tank have an air stone or other type of aeration? no Is your tank heated? Yes . Eheim Jager 100 watt. What tank mates does your peppered cory live with? Albino, emerald, and melini cory catfish. One rubber lipped pleco. Food What type of food do you feed your Peppered Cory fish? Omega one Veggie Rounds, API Bottom Feeder Shrimp Pellets. How often do you feed your Peppered Cory? I give him between one Veggie Round in the morning with his tank mates and one in the evening. Also put 3 bottom feeder in the morning and 3 in the evening. Maintenance How often do you perform a water change? Weekly change on main tank every Friday or Saturday. Last change was today. What percentage of the water do you change when you perform a water change? I take out approximately 70-80% of the water. What type of additives do you add to the water when you perform a water change? API Stress Coat + 10ml for my 20 gallon tank. API Aquarium salt - 1 TSP per 5 gallons added. (reduced from last water change of 1 TBS per 5 gallons). Latest change today (7/17) I added no salt. 1-2 ml of Seachem Prime 2x per week. First dose on water change day and second dose on Wednesdays. Water Parameters: Have you tested your water? Yes. From API Freshwater Master Test Kit Note: I did an early water change today because the fish seemed really lethargic. I did a water chemistry check and discovered the nitrites and nitrates were very high. Last two water changes I've been using doing a hydrogen peroxide bath before each water change following these instructions: After looking at the numbers I suspect I might have killed off the beneficial bacteria in my tank. The other thing is that I noticed a lot of uneaten food in the tank during the cleaning. The food was floating around with a dandelion like fuzz around it. I siphoned the tank with a gravel vac and also vacuumed the tank some more once the water change was done. Not sure why there was so much uneaten food. I don't feed these guys a ton. First set of numbers before the water change today: PH: 7.2 - 7.6 Ammonia - 0 ppm Nitrite - 5.0 ppm (very bad) Nitrate - 20 ppm (unbelievably bad) Hardness: I don't have a way to test this. Alkalinity: I don't have a way to test this. I did a 70% water change on 7/16 (the fish looked stressed so I didn't take out any more water). After the water change, here are the parameters. PH 7.2 - 7.6 Ammonia - 0 ppm Nitrite - 1.0 ppm Nitrate - 5.0 ppm I changed the water today 7/17/15. The number are now: PH 7.6 Ammonia - 0 ppm Nitrite - 0.25 ppm Nitrate - 0 ppm *note: I can still see some food coming up from the substrate when I siphoned the tank today. I think I need to do some more deep vacuuming. Symptoms and Treatment How has your Peppered cory's appearance changed? Yes, I noticed this dark purple bump below under his mouth. How has your Peppered cory's behavior changed? He appears to be eating normally but not a lot of movement. He just sits on the bottom and breathes, with occasional position change. He's hiding under vegetation. When did you start noticing the symptoms? Yesterday. Have you started treating your fish? If so, how? I have not started treating him. Does your fish have any history of being ill? Nope. How old is your fish (approximately)? No idea. I've had him since December of 2014.