Do the water change tonight. What if you had poison ivy all over your body, and there was no Calamine? That's what it probably (I've never been a fish, I'm just guessing here) feels like. You're up. You're typing. You're coherent enough to do a water change! (I'm not a physician either, this just seemed to make sense.) Oscars lose about half of their food through their gills. Have you ever watched them eat? It's really quite gross. I'm not sure how they reach the sizes they do, when most of their food doesn't reach their stomach. Your aro and catfish are eating, I'm sure, and pooping, and adding to that bioload. What type of filtration are you running? If it's HOB, it's likely not doing enough to deal with this. How often are you gravel vacc-ing? If it's not weekly, with the Oscars, you should be doing it weekly. Variables do exist here. Filtration, amounts fed, frequency of feeding, but what you describe is ammonia burn, and it's hitting the catfish first. Since you have no current readings, I'm going to go with this. Do a huge water change tonight, please.