I have a 125 gallon tank. The occupants are 2 oscars, 1 green terror, 1 parrot, 1 tinfoil barb, 2 silver dollar, 1 convict and a pictus catfish. The tank has been up and running for about 6 months.

PH is 8.0
Amonia 0
Nitrite 0
Nitrate 10
Temp 76

My problem is that one of my oscars ( 5 inch red) has been sitting on the bottom of the tank for days now. All of the other fish in the tank are VERY active. The oscar who sits on the bottom has not been eating very well. He eats probably 1 out of 3 times I feed the fish. When he does eat its not with the enthusiasm of the other fish. Very odd considering oscars are usually huge pigs. The other oscar certainly is.

There are no visible signs of stress or problems with the fish.

About 5 weeks ago the tank had ich and I treated it with salt and heat and coppersafe. I have had no signs of Ich for at least 4 weeks now. No fish rubbing on things and no white spots. Since then I have done a few 25% water changes and used carbon.

Could this fish have some sort of internal parasite or infection?

I'm sorry but it's really impossible to say whether your fish has internal problems. Normally their abdomen becomes distended if they have internal parasites or bacteria infections. Have you tested your tap water for pH? You wouldn't want your pH to get much higher. Have you considered putting some bog wood in to try and lower it is a bit? Is the fish in question breathing heavily at all?

I have to say that it is not uncommon for Oscars to suddenly stop eating. It's very difficult to give you a reason why they do it unless there are obvious signs of illness. If this is your case, all I can suggest is keeping your water in good condition. You could also try feeding a different foods, maybe some shrimp or krill, or even mealworms. You never know, the change of food might just trigger something.

How often do you do water changes? I have to say that your nitrate is quite low considering you are slightly overstocked. Tinfoil barbs can reach 12 inches. I've also got a 125 gallon tank with two Oscars and four clown loach and my nitrate level is normally around 30, sometimes 40 at the end of the week.

yeah your nitrate is low for your stock,
i have 2 oscars, 1 jack dempsey and 3 clown loaches and my nitrate today was 30.
what water test kit do you use?

my fish did the same thing when i added other fish, he began to stay in 1 corner and not eat or move,sometimes i think they feel over crowded and will get weak , then the other fish will pick on him.try putting him in his own tank and see how he does, or a tank with not so many fish, just like people some like crowds some dont:)once i removed my 1 fish everything went back to normal with him. good luck!!! twinkletoes

I'm wondering if it's leftover Ich. The reason I say that is mine would do the same thing (pout essentially) but not show any signs of ich, however after a round coppersafe or quick cure they'd be back on there feet. Err fins. Whatever.

Anyhow, seeing as how you just had a case of Ich roll through, it might be worth considering another dose of coppersafe if he doesn't perk up.

Remember there is only a specific point in the Ich cycle that meds will address it. Raising the water temp increases that cycle so meds are more likely to be effective if you raise the temp. Don't forget to pull your carbon out while medicating too (if you're using it).

courtesy peteducation.com:
"This stage is very temperature-dependent within its capsule, with the fastest replications occurring at warmer temperatures near 78-80°F. At optimum temperatures, the replication will be completed in about 8 hours. At lower temperatures, the replication takes longer making the treatment time for eradication much longer."

My other thought is... that's a lot of fish for a 125. As twinkletoes said, he could feel cramped is is pouting over that. Mine pout for a while after every tank cleaning, and they've been going through that for years!

when did it start that behavior?
for the meantime, i suggest you go 40% water change with out cleaning the filtering system. pristine water is the first and foremost defense of fishes against any illnesses.

gradually increase the temp to 80 deg (gradual means 80 in 2 or 3 days)
i suggest add a dose of pure salt (1 tbs/25 gal water) to soften the water and to enhance oxygenation.
add a little stir in the surface of water to add aeration.
hope this helps.

My test kits are all Aquarium Pharmaceuticals except for PH which is a a Tetra.

My LFS suggested I try Kent's Garlic food additive on their food to see if he would be enticed to eat. I've done so a couple of times now and as usual all the other fish eat just fine but the red oscar sat on the bottom never moving an inch with food going past his head.

I think my nitrate is low because I had just done a couple of water changes in past week.

I retested though to be sure.

PH 8.0
Amonia 0
Nitrite 0
Nitrate now 20 (This is a guestimate of course but its closest to the 20 Color)

I did rearrange the tank in the previous week and added some new rock.

I'm considering taking the O out and putting it in a quarantine and dosing something like clout.

unnecessary movement will stressed your oscar.
i suggest you observe him a day or two before
putting it on a hospital tank if no improvement.

Well the sick oscar died early today. I woke up and saw he was unable to stay upright and was doing sort of a barrel roll. I went back to check about 15 mins after I noticed him doing this and he was dead.

So whatever was wrong killed him fairly quickly. I just hope its not going to present in the other fish. As of now all the other tank inhabitants are healthy and eating.

Sorry to hear about the fishes death, it's never nice losing one of these fish

sorry for the loss.

i suggest you carry on the water change to lessen any possibility
of contaminants present in the tanks. water change will keep your
water under check.

did i make you decide not to move the fish in your hospital tank?
yes, i did suggest. if i did, i am very sorry for mis-judgement.

oh nah I dont think it would have mattered. Whatever the fish had killed him fairly quickly. Thankfully none of the other fish seem to have any symptoms. I'm going to keep a close watch though.

Thanks