Mystery illness on zebra loaches

[neppy]

I have a large (75g) tank with 3 zebra loaches, yoyo loaches, and a few assorted catfish.

About a 2 months ago one of the zebra loaches stopped eating and lost weight. After isolating him in a hospital tank and trying treatment for parasites I had to euthanise him.

Now the remaining 2 zebra loaches are starting to show weird symptoms:

-general lethargy/lack of energy/lack of interest in food
-somewhat veiny appearance on skin
-one of them has a single badly discolored patch on each side of its body near the rear fins (not the tail)

Help? If I can't find out what to treat these fish with im pretty sure theyre going to die in 1-2 months like their brother. All the other fish in the tank are fine. No new fish have been introduced into this tank for almost a year so I find it hard to believe I introduced a new disease.

[Diana]

What color is the 'badly discolored' patch? Grey-white suggests Flavobacteria columnaris. Gold suggests Velvet (a parasite). Red suggests and internal bacterial issue, or bruising following an injury. Fuzzy and white suggests fungal infection, usually at the site of an injury.
You can google each of these ideas and find pictures, see if any of them look similar to what you are seeing.

The other symptoms (Lethargy, poor appetite) suggest a problem, but are not specific for any one disease or problem. Skinny hints at intestinal perasites, but is also a general sort of symptom. Fish that are not eating for whatever reason will lose weight.

Have you treated these fish for intestinal parasites? Do a search here at Loaches with the word 'Skinny' and see some of the issues that are related to this.

Diseases and parasites might always be present in our tanks, but healthy fish have a strong enough immune system to fight off these things. When there is some stress such as poor water conditions, or social issues then the immune system is weakened and the fish can get sick even when there have been no new additions to re-introduce disease organisms.

Can you test the water for all possible things that you have tests for?
Increased water changes can help: Lower the nitrates, and reduce the organic matter in the tank. Several bacteria thrive with poor tank conditions.

[neppy]

columnaris is the only thing that looks remotely the same but even that is a stretch. I'll increase water changes for quality and treat for it.