It's most likley a direct competition for food. I agree it's hit or miss with the 6 line, stupid fish and they're personallities. A lot of people have them with no aggresion, but IMO, both those two fish are rock grazers, and directly compete for food. The 6 line gets tired of old goby eating his pods and like a little pinstripe suited pit bull, SNAPS! Larger tanks with more rock and you probably wouldn't ever see this. Same tank with different individuals and you may not. Through the dice man, you could have a customized goby too!