I have, on numerous occasions, witnessed this and wonder if others have as well. First, let me tell you about my oscar. It is nearly 18 months old and has a voracious appetite. Any food dropped in the tank will immediately be devoured. But, if I remove one of it's roommates or do an extra long cleaning of the tank (just taking longer not changing more water), something strange happens. It is as if the oscar doesn't recognize or trust me anymore. The exact same food offered the day before will now be refused. Oh, the oscar will sample it, but will immediately spit it out. It does not matter what type of food, either. This has happened with pellet food, green peas, human-consumption cooked shrimp, etc. However, the next day, the oscar will resume it's normal feeding behavior. It is as if the oscar's memory has been completely erased, but is fully restored the following day. I have read that some peoples' oscars sulk after a tank cleaning, but mine does not. It will continue to visit all areas of the tank, and all behaviors are the same, with the exception of feeding.