RES question?

you could try feeding him outside of the tank like in the bathtub many slider owners do this as you will know their water gets dirty very quickly and feeding them outside helps with this issue he also needs more room it can be very distressing to be kept in a to small area. make sure he can get out onto a basking rock or turtle dock you can even try hand feeding whilst he is out of water (use tongs)

Its because Gorbie needs a bigger aquarium but since its going to take you some time to buy a new one do as suzie said: feed him in the bathtub. If you dont mind here is some advice, my cousin had an res which was around 7 inches and he had the exact same problem so he traded it for 3 small res. If your not very attached to your turtle you can do that, but if i were in your place i wouldnt. There is no way that you can teach him not to be aggresive, since you fed it live food it sort of became wild and i guess its too late to change that but its ok because turtle are usually not very friendly like a cat or a dog.