I see 2-55gls. I hope you have tons of live rock and live sand with refuge. 4 years is good but CBBF feed mainly on micro orgs like tiny worms, pods, mysid, ect. that live and hide in the rosk. Just because its looking does not mean its finding food, the orgs have to be there.
You can try live brine or worms but the fish will not live long on just those.