Let me start out positive before I get the hatred my way. I agree with
Stanzzzz7
a 6x2 is not nearly big enough for an adult Dovii. I think he hit the nail on the head 8x3/ 8x4 being the minimum for a full grown adult with 10x4x4 being my ideal for a pair. (Just my opinion)
This is why so few of us have full grown Dovii and why my heart weeps everytime I see some one with a stunner male.
Now for the hatred part. As for grow out tanks, I believe that as long as the fish can turn around with plenty of space then it's fine for temporary housing. So I'd say your fine untill it hits about the 10 inch mark or so. One of the problems I feel your gonna run into is the length. He is gonna get mad salty in that short tank. That's why I like to keep big grow outs in a 6ft footprint. They use every bit of it.
Dovii get huge, beefy, and mean. Commonly hitting the 24 inch mark in captivity. And I've read that they can get even bigger then that.
The other problem is the 18inch depth, I know we don't talk about depth much. But what type of lid you have on the tank? I've seen Dovii attack top water pretty hard core. I've also seen first hand Dovii that pushed over glass lids and met their death on the living room floor because of it.
But all in all I think he is fine for now, start thinking about an upgrade soon their fast growers. Good luck and keep us updated. Those of us that just can't keep massive tanks love seeing those fish!