I've just got the tusk and a chainlink eel right now. They get along great, the tusk thinks the eel is his best friend but the eel could care less about him. Not that the eel is mean to him or anything, he just doesn't try to hangout with the him like the tusk does.
I have a HOB
filter right now and the eel uses it as his own personal little tank and when he decided to come out the tusk gets so excited and swims over to him and huddles up next to him and stuff.
There's pics on my profile