Nice terrors! The blue on that male is very vivid! At 6 inches he doesn't look very beefy as you say! Feed him the meaty stuff, but mix it up with the usual also. For the reason you listed above. I don't mean to doubt you, but are you sure they are 3" and 6"? They just look smaller thats all! It may be due to the parasites. They haven't yet achieved their adult colours yet IMO, they will continue to gain colour - especially the orange on the fins.