Is my Black Widow Tetra going to DIE?

[Lyds001]

Hi, I have a a 135 liter tank with 20 fish of various species (all tropical). Four of those fish are Black Widow tetra. Two of said tetra has strange black marks on them, I’ve looked closely and they bulge out of the fish slightly so I know it’s not the colouration of the fish.
I’ve tried to catch those two fish to get them into a quarantine tank but I was unsuccessful and I didn’t want to stress them out too much and make the problem worse.
I’ve treated the whole tank with the eSHa 2000 FUNGUS, FINROT AND BACTERIA TREATMENT multiple times but it hasn’t gone away. On one of the fish it’s in a large lump on its side, on the other it’s all around the face, eyes and gills, there are also a few black dots on his fins and body. The fish with the marks on his face has recently started to swim on his side, which doesn’t seem normal.

Here are some details about the tank that might help:

Temp: 29 Celsius
pH: 6.5
There is drift wood (and therefore tannins) in the water
I do 25% water changes every two weeks

I hope someone could help me, I don’t know what else to do.

 

[King Fisher]

Generally turning black is a neurological disease. Swelling in the stomach region is more than likely an internal parasite.

25% water change every two weeks isn't nearly enough. Start 50% weekly water changes as a regular routine. I would immediately do back to back 50% water changes.

What is your ammonia, nitrite, and nitrate readings?

How long has the tank been running?

What kind of fish do you keep? 20 could be to many for your size tank depending on what they are.

Why are you keeping the temperature so high in your tank?

 

[Lyds001]

I have 4 black widow tetra, one angel fish, one Molly, 6 neon tetra, 4 golden barb, two bristle nose plecos and two more tetras.

If that’s too many I do have a spare tank but it’s only 25 litres.

The guy in my local fish shop told me to have the temperature to 29

The tank has been running for about 2 years

I don’t know about ammonia and nitrite but I doubt they’d be high since I havnt added any fish in ages and my filter is really old with loads of beneficial bacteria.

 

[King Fisher]

I'd lower the temp to 26C. Some of your fish won't handle 29 over extended periods of time.

Your tank is definitely maxed out. I'd change out 50% water weekly for sure.

Did your filter dry out at any point? Did you keep an ammonia source in your tank when you didn't have fish? BB needs ammonia or it dies. BB also dies in filter media if it dries out or if it's left out of water for sn extended amount of time.

 

[Lyds001]

I’ll lower the temp, thanks for the advice.
I thought the fish in the tank supply the BB with ammonia...
I’ll start with the 50% water changes
The filter has never dried out...

I don’t know what to do about the black marks... is it a parasite or a fungus..?

 

[King Fisher]

Sorry, I thought you said you didn't keep fish for awhile in the tank.

Fungus is usually white. Fish turning black a lot of times is a neurological disease. I'll look into this a little more though.

 

[Lyds001]

No sorry I meant I haven’t added any new fish in the tank recently.

Thank you for helping me