I did read the link, but not sold on it being the only option (especially given that the fish in the thread died).
Think i replied to your other post. Didn’t see this post just from scrolling till tagged when i replied to your other post. I’d try feeding several time per week. Try a tablet with more veggie in it. I made a suggestion of treating with oral metro, levamisole and kanamycin. Id suggest just the metronidazole and levamisole for now. Mix some pure epsom salt with its pellets. Sometimes the Epsom salt with pellets is enough or just adding occasionally. Monitor its poop for color and how often you think it’s going. If all else fails and 2 things left to try, changing your tank into a black water tank (slow transition and maintaining a stable ph always). Seen it with other oddball knives and certain catfish I’ve kept. Keep in the regular Fl hard water and die and get sick. Change over to a soft and acidic water and start to thrive. Change over to a black water tank is the one thing that hobbyists have a hard time to do, i get it. Last option is see a vet. Best of luck and hope things work out.
Been trying to contact a vet, but do not think those really exist here for fish. And if they did, it would likely be EXTREMELY expensive (thousands of dollars).
I have a medicine with "Levamisole HCL" as the only active ingredient. Is that it?
Why Levamisole?
If the op has had this fish for 6 years, and nothing new has been recently introduced, the chance of this oscar needing a dewormer would be pretty much zero. I would be more likely to lean against a blockage from constipation, or possibly an internal bacterial infection.
If the op has had this fish for 6 years, and nothing new has been recently introduced, the chance of this oscar needing a dewormer would be pretty much zero. I would be more likely to lean against a blockage from constipation, or possibly an internal bacterial infection.
…. and I would start small, and only elevate to meds if epsom salt doesn’t improve his condition In several days. You can safely treat the main tank water, and presoak his food. No issues, no idea why anyone would say it might cause harm, or no proof on this helping a fish that may be constipated. Ridiculous.
Just covering for any possible causes. Makes sense. If treated with dewormer and not better then less likely of parasites. One cause ruled out. But can go the Epsom salt first and try the others later in step fashion.
Fair enough, but Levamisole is only effective for worms, and only specific worms, so unless there is an indication of worms in this set up, I would personally start small. Doctors are always so quick to hand out prescriptions! Lol. JK. 
Agree
If no new fish or plants have been added, the addition of parasites would be negligible.
OP You give pH, but not water hardness or other data that may lead to chronic problems with soft water species, and that may take years to slowly develop.
Bacteria can develop under certain mineral conditions and cause chronic disease, they are ubiquitous in the right water conditions
Is your water from a well that could be high in iron, or some other mineral,?
That ...in concert with 10-20ppm nitrate before a water change may be a causative soup.
Beside trying the epsom salts, I'd double up on frequency ,and volume of water changes to see if things improve.
That water change schedule may have been adequate when the oscar was young, but I don't consider a 35% to 50% water change once per week adequate in that size tank for an adult oscar with other tank mates.
Agreed, especially with a Sailfin pleco and friends as tank mates. That’s a hefty bio load for a 130 gallon tank.
