First off, one Fluval 305 isn't enough filtration for a 75 gallon tank. It is only rated at 265 gph and the minimum is at least 5 times the tank volume which would be 375 gph. I am running 2 304s (the previous version) on my 75 gallon tank. For a 4' tank i ersonally think it is better to have dual filters and heaters to provide better circulation and temperature control.
The only media I use are the 4 sponges and the ceramic rings although I did add some of the Matrix Bio-Max too. I don't use carbon as it has to be changed too often and is primarily a chemical filter anyway. I would stay away from any type media that says it "removes ammonia, nitrite and nitrate". If it removes ammonia and nitrite, your tank won't cycle as there will be no ammonia and nitrite for te bacteria colony to feed on. And if it is removing nitrates, you'll never know if ammonia is being processed properly as there will be no nitrates. Also, if you have live plants, they will need nitrate to feed on.