welcome to fishlore!!!
Hows the oscar today? I hope better!! although I agree its probably settleing in, I also think like nick said that 2 plecs in that size tank could produce a ton of waste aka
ammonia if the tanks not cycled... , im wondering what are your water partimeters for ammonia,
nitrite, and
nitrate? partial water changes with some prime always make fishies feel better....and id also think about returning one of the plecs so you dont have to do every other day water changes in the future