Since he signed off, I will answer this question for you. For proper maintenance you are supposed to do at least a 20% water change each week. However it's not the end of the world if you don't. Pacu's and Oscars are rather hardy fish. So they will tolerate the water conditions well.

Depending on the type of media in the filter you will need to change or cleanout the filter at least once/month. However you do not want to mess with the filter to often because some of the bacteria on the filter is good for the fish.

Check the ph levels periodically. Once again pacus and oscars are very hardy fish. I've seen many of them thrive in water straight out of the sink. So although it is a good idea to check the ph levels to keep the fish from getting sick I would not say it is a necessity to do it on a regular basis.

Outside of feeding, that about sums up the maintenance. Most freshwater fish are pretty easy to take care of.