Help tetra with black growth on bottom

hello,

I have a 7yo tetra with a black growth on the bottom of it, I have no clue what it could be help
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Tank
What is the water volume of the tank? 20 gallons
How long has the tank been running? 7 years
Does it have a filter? yes
Does it have a heater? yes
What is the water temperature? 75 degrees
What is the entire stocking of this tank? (Please list all fish and inverts.) 5 harlequin rasboras, 1 glofish tetra, army of bladder snails (yeah I know pretty low stocking havent been able to get fish for a while)

Maintenance
How often do you change the water? Every 2-3 weeks
How much of the water do you change? 50%
What do you use to treat your water? Aqueon dechlorinator
Do you vacuum the substrate or just the water? I vacuum the substrate

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water?
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 5ppm
pH: 7.2
GH: No test kit
KH: No test kit

Feeding
How often do you feed your fish? Once daily
How much do you feed your fish? medium pinch
What brand of food do you feed your fish? Tetra flakes
Do you feed frozen? Sometimes
Do you feed freeze-dried foods? No

Illness & Symptoms
How long have you had this fish? 7 years
QT Used….Yes/No, ……if Yes, how long? No
How long ago did you first notice these symptoms? Today
In a few words, can you explain the symptoms? There is a black growth right in front of the analfin and a clear ovaloid sack below it that looks full of fluid
Have you started any treatment for the illness? No
Was your fish physically ill or injured upon purchase? No
How has its behavior and appearance changed, if at all? It hasn't been moving a lot but I suspect that is because of old age as hes gradually been slowing down


Explain your emergency situation in detail.
(Please give a clear explanation of what is going on, include details from the beginning of the illness leading up to now)

There is a black growth right in front of the analfin and a clear ovaloid sack below it that looks full of fluid which I noticed tonight. He is still eating and swimming fine so I don't think it's infecting him much I just wonder what is it

Include pictures
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Looks like a prolapse from what I can see.

Looks like a prolapse from what I can see.

Click to expand...

How would I treat it? It just came suddenly yesterday, and I have no idea what caused it.

Does anybody know how to treat? It doesn't seem to be affecting him nor does it seem to be getting worse so maybe it's some sign of old age? He doesn't eat as much as he used to but he's been doing that for months now.

At seven he's old for a GloFish tetra (they're black skirts underneath), and eating less for months before this showed up makes me think something inside is pushing it out, like a growth, rather than a plain prolapse from constipation. There isn't much of a treatment for that. Epsom salt at about a tablespoon per 5 gallons can bring swelling down. Watch whether the rasboras start picking at it. If he's still swimming and eating, I'd leave him be.

Thank you! The swelling seems to be going down a bit on it's own but I will give him a salt bath. The rasboras are fine with it.