My FRT was the same way until I tried keeping it with gars. Others have reciprocated that there is something about the gars that draws the FRT's interest. Likely the fact that they don't move much and rest on the bottom.
Try it if you want, but I thought my FRT was a gentle giant as well (still is with other fish) but he hit four gars in about a week and I'm not talking about frayed fins, I'm talking about half of tail fins missing.