Bloating and Large Cyst on Tetra

Ammonia 0
Nitrite 0
Nitrate <10
pH 7.3
gH 200 ppm
Chlorine 0

29gal, moderately planted, gravel bottom with 15 cardinal tetras, multiple cherry shrimp, a clown pleco and two honey gourami. I do a 25% water change every week, water temp is 76F.

The past month I have lost three fish, one gourami and two cardinal tetra, with no visible signs of disease or behavioral change. This may be unrelated, but I think it's important context.

The pictures are pretty good imo but I can try for better ones if that would help. There seems like two cysts mirrored on both sides of the cardinal, with some noticeable bulging in the torso between them. There are no behavioral changes in the fish, and the school hasn't shunned him. Am I right to think this isn't ich? It seems too large and not white.

Cardinal Tetras have a tendency to have better immune systems than Neons. You would want to quarantine the fish and do a large water change. Try get all the gravel with a gravel cleaner as the root will be in there, No matter the cause.

It doesn't look like white spot.

If it's on opposite sides of the same fish and the fish is getting fatter, it's probably an internal parasitic worm or bacterial infection.

You can try deworming the fish but it might not work.

If the fish stops eating and does stringy white poop, then euthanise it.

Colin_T said:

It doesn't look like white spot.

If it's on opposite sides of the same fish and the fish is getting fatter, it's probably an internal parasitic worm or bacterial infection.

You can try deworming the fish but it might not work.

If the fish stops eating and does stringy white poop, then euthanise it.

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OK, if I deworm and it does nothing, what would the next step be? Assuming it's bacterial, is there any way to determine if I am I going to be looking at a tank-wide infection or an isolated case?

It will be an isolated case because it is coming out of both sides of the same fish and no other fish appear to be affected.

If you deworm the fish and there is no improvement, then you either euthanise the fish or let it live until it stops eating and then you euthanise the fish.

See section 3 of the following link for deworming fish. Flubendaolze would be the wormer of choice. If that is unavailable, then Praziquantel and then Levamisole.

Update: I looked last Friday and seeing the fish front to back, I noticed it looked like a puncture wound, like the two spots were ends of something that speared into the fish. There was a slight diagonal angle visible too, so it wasn't straight across. I had to be away for the weekend and when I came back one side no longer had a protrusion. The dark internal spot was still present but the other protrusion was paler.

Colin_T said:

What to do if your fish has Stringy White Poop.

Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...

www.fishforums.net

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I haven't been able to see any poop, but he is eating. I bought the medication you mentioned but haven't used it yet because I've been away.

Looking at it again, I remember a previous case similar to this in my tank. A cardinal had what looked like a shard of gravel or shell stuck inside it, but whatever it was eventually fell off and left a scar. It lived for about a week then it passed away.

Does this change anything? Do I still need to use the medication?

I would still try deworming the fish. At the very least all the fish will be free of worms afterwards. The fish in question could still die but deworming is safer and not going to cause problems, unlike many other medications that probably won't help anyway.