Oscar sick or sulking?

Hi,

Recently my red tiger oscar has started just sitting around like he is
sulking. I can usually con him to come out with food when he is
sulking, but lately he just ignores everything. His poo's are
floating, so I was wondering if it's possible that we've been
overfeeding him lately? He started sulking right after I did a huge
water changed (80% of the water) because the tank had some bad algae
going on (that floating strandy type algae), and really to take out
half the water wouldn't have made a difference. I also put in some
anti-algae stuff and I'm pretty sure I didn't overdose the tank.

Other than sitting around not eating, he is his usual charcoal colour,
and hasn't gone grey around the gills or anything else. When I pay
attention to him he sort moves a little in his spot, and moves his
mouth a bit, but that's it!! The only other thing I can think of is
that he had a small red christmas bauble as a toy tied to his rock,
which I threw away, could he just be sulking because I took his toy
away? He never seemed to play with it often anyway...

What's wrong with my oscar?!