H. g. will eat amphipods, but you may need to suppliment with an occasional small piece of raw shrimp, krill, etc. The typical H. g. is about 1-1.5 inches long and should be given a piece of food not larger than an 1/8" cube - perhaps up to a 14" if it is approaching 2 inches.
They like to take things out of the water column, so live adult brine shrimp also make good food.
Roy