Hi and welcome Lethargic8.
Do you have test kits for ammonia, nitrite and nitrate? If so, can you please test and the post the results? What is the temp of the tank? How often do you do water changes and gravel vacs? Any tankmates with the oscar?
The Oscar has finrot, this is a bacterial infection that is usually caused by the Flexibacter columnaris bacteria. This type of infection can be brought on by any number of factors such as:
- poor water quality (ie- a cycling tank, nitrates over 20ppm, poor water change frequency)
- fighting/fin-nipping
- newly imported fish or fish that are new from the store
- pH shifting greater than 0.2pH points every couple days
Signs of finrot include:
- redness around the base of the spine of the fins
- ragged, torn or stumpy fins, sometimes with a white or greyish edge to them
- disintegrating fins
If the oscar is the only fish in the tank then you can treat him in the main tank, if not then he needs to be quarantined. Finrot can be treated using a combination of Maracyn and Maracyn 2. The Maracyn is erythromycin and will treat for the finrot, the Maracyn 2 is used to treat for any possible secondary bacterial infections that may be present.
Lower the temp of the tank to 76F, this will slow down the spread of infection, at war mer temps this will spread rapidly. Treat the fish for five days, after the fifth day perform two large water changes twelve hours apart, up to 50% each time is fine, then treat for another five days. Remember that is important to treat for a few days after all signs and symptons of infection have disappeared.
These meds will lower the dissolved oxygen level in the water. When using them you will need to lower the water level an inch or two belowe the water return of the filter or add an airstone to the tank to keep dissolved oxygen levels up. They will also cloud the water, this is normal and nothing to worry about. They are also photosensitive so lights should be dimmed or kept off. If you run carbon in your filter you will need to remove the carbon.
HTH,
Bryan