Yesterday's water conditions:
Nitrate 0-5.0
pH 8
Nitrite 0
Ammonia 0

Oscar was provided his own 55 gallon tank after being injured in a lip/jaw lock fight with other oscars. His upper jaw is injured, but he is able to eat. He shares his tank with a mono and a pleco. I have a live plant in the tank. I also have bubbles and perform weekly water changes.

Everything seemed fine until yesterday when I found him floating on his side, barely breathing. After testing the water, the only thing I could think to do was to lower the pH and add 2 Tbl salt. (I add salt at all water changes, we have very hard water.)

Within 30 minutes he was swimming again and seemed normal.

This morning, he was swimming and seemed normal. About an hour ago, I went to test the water and found him floating again. Everything is the same as yesterday, but the pH is now down to 7.6. Knowing pH can fluctuate from night to day, I'm thinking the pH may be rising, although I have a lower reading. I added pH down again and 2 more Tbl salt. No change in his behavior...still floating, shallow breathing.

Any advice?

I doubt very much that it is the pH. Have you checked your pH coming out of your tap? Altering the pH artificially could make unstable so be careful.

It really is impossible to say what could be wrong. If the fish opening and closing its mouth constantly? Has it stopped eating?

Thanks -- no I haven't checked the pH from the tap...will do. I didn't want to alter the pH any more, for fear of making it too low, and giving the fish stress from pH fluctuation.

He is swimming normally now, I just tried feeding him. He looked at his pellets, but didn't eat.

No, he's not opening and closing his mouth constantly, even when he is floating on his side. When he floats, it is slow, shallow breathing, not the fast gasping.

I am absolutely baffled. If I hadn't see him for the period last night or this morning, I'd never know. That being said, I have no idea how long he's been doing this. Except for the not eating, he seems perfectly normal. Last night when I found him floating, I thought he was certainly about to die.

Anything else I could check?

Difficult one. Is there any obvious signs of swelling anywhere on the body? A swim bladder disorder could make a fish difficult to keep its balance. If your fish continues to refuse food then that could be a cause for concern. Any creature that is ill will go off its food. I haven't really got any suggestions apart from monitoring the situation and seeing if things improve or get worse.

pH from the tap is 7.4-7.8

There are no external signs of any illness, scales, eyes, fins, everything looks fine. No swelling, no signs of constipation. (Unlike one of our gouramis who will be getting an epsom salt soak later today.) All signs are behavioral.

Thank you for your input. I will continue to monitor and post. Please let me know if you, or anyone else, can think of anything.

hard to diagnose! but be gentle in salt application.
if your are not doing water change, do not add more
salt. too much concentration will also put your fish
in distress. there must be something also that triggers
sudden fluctuation on your pH. perhaps the injury still
affect it behavior.

Thanks for the salt warning. The fluctuation in the pH was caused by me adding pH down, thinking this particular fish didn't like pH of 8??

He ate last night, and is swimming normally again.

water conditions:
Nitrate 0-5.0
pH 7.6
Nitrite 0
Ammonia 0

I have made no adjustments since yesterday morning's addition of salt and pH down. As of this moment, the only theory I have is that this particular oscar is sensitive to pH over 7.6.

I'd appreciate hearing any other theories.

oscar can tolerate pH between 5 to 8. 7 is the normal
what distressthem is the sudden fluctuation. gradual
changes willhave no effect on them.

you should try to figure out what causes the erratic
fluctuation or sudden change of your pH to avoid occurence
of the same problem.

Thanks for the advice. The downward movement of the pH is due to me adding pH down. I don't know why it won't stay down. I am thinking I may need to add more live plants.

This morning, inbetween swimming and floating. Checked water:

Nitrate 0-5.0
pH 7.8
Nitrite 0
Ammonia 0

Added pH down again. pH at 7.6, and happy swimmy fish. He ate breakfast.

The mono and pleco are both fine through all the pH fluctuations.

At this point, I'm going to assume this fish is pH sensitive and will not tolerate pH higher than 7.6. Unless there are any other theories?