Bit of an update, all forms of feeder failed...............
The good news is, by turning the powerheads off and feeding bluey (my mandarin) under an overhang while everyone else ate above, he seems to be putting weight back on. This way he seems to get 10-15mins to eat before the other fish move down to where he is.
I'm alternating between mysis and brine shrimp with my feeds and am soaking in Garlic Concentrate (after rinsing the shrimp cubes).
The line of bone behind his fins is almost gone. Fingers crossed he keeps eating and becomes a fat happy fish.