Unfortunately, this oscar is not very well right now. He spends most of his time standing on its head and turning like a top. And he hasn't been eating for more than a week now.
A friend of mine said that it was most probably swim bladder problems due to bacterial infection resulting from a high level of nitrates.
But I don't think that this might be it. I'm more convinced that this results from the jump he made 3 nights ago. He must have hurt his swim bladder when he dropped to the floor.
I'm not sure nitrates are high in the tank. After all, I do 50-75% water changes once a week and I've got powerful filters. And he is not overfed.
Ah well, anyway I 'm planning to do a 75% water change every 2 days and will be adding antibacterial medicines. We'll see what happens then.