One of my yellow tail Congo tetras (a. caudalis) looks like he was body slammed. See the opaque area on his posterior, dorsal flank and what looks like a pink abrasion on his anterior, ventral flank. Is a combat injury or an infectious disease?
I just noticed the markings today. There is nothing new in the tank. Thanks for the information.How quickly did it appear?
Have you added anything new to the tank in the last 2 weeks?
Is there anywhere the fish could have gotten stuck?
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The white bit in the muscle tissue could be a parasitic worm or a microsporidian infection.
The red on the belly looks like a bacterial infection.
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If there's a worm in the muscle tissue you can normally kill it by using Praziquantel or Flubendazole.
Use salt (sodium chloride) to treat a microsporidian infection. Add 2 heaped tablespoons of salt (rock salt, swimming pool salt or plain sodium chloride salt from the kitchen) per 20 litres (5 gallons) of water. Keep the salt in there for 2 weeks.
The bacterial infection might respond to salt (same dose as for microsporidian). If it doesn't improve after 24 hours or gets noticeably worse, then you will need a medication to treat bacterial infections (preferably not an antibiotic).
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Before treating the fish with anything, wipe the inside of the glass down. Then do a 75-90% water change and gravel clean the substrate. Make sure any new water is free of chlorine/ chloramine before it's added to the tank. Clean the filter. Then add salt.
You can deworm the fish in a couple of weeks time if it survives the bacterial infection.