Agree, this would be the #1 defining factor in what all OP could "potentially" add in as a tank mates of sorts. If it's on the small end like a 55g I would recommend either a striped rapheal catfish or some nerite snails, or if it's on the larger end like a 200g I would then recommend either a school of mascara Barb's or maybe even a couple more cichlids such as a convict or a salvinni. Also another option, if your not against occasionally buying more fish, you could add in a big school of zebra fabio's, just know that they will slowly get eaten up so you would need to occasionally buy more.
Just to add. I tried parrots, plecos, silver dollars and nothing yet worked for me.
Yeah sadly that's the case with most FH's. That's why I would mainly go with species that are either fast enough to escape or tough enough to handle the abuse. Also I find that having something that's nocturnal can also work just due to both fish being active during different times.
