It'll be fine.
It even wouldn't matter how you carried the beam to the corner post. The beam is being cantilevered via the 2 load bearing post. Rule of thumb is to not exceed more than double on one end than you have on the other. Or 66% to 33%. Which you haven't.
So, your 2x6 beam will suffice even without the corner post. Your 2 posts placed within the span of your beam is carry most of the load...
Just be sure the ply panels are securely fastened. These are crutial, as they provide shear strength. It keeps the frame from racking and potentially folding up on itself.
Glue up of the panels would be great if possible. If your just nailing or screwing the panels on, try placing them every 8 inches around the parimeter and 12 inches in the middle. Don't go too much. It'll actually weaken the panel. But then you chose 3/4" ply which is Really strong for this application. 1/4" would have worked. So your all in the green there too.
But if you are concerned, you can sister on another 2x6 behind the existing one. This one wouldn't need to be carried by a post at all. It would be there just to stiffen the first 2x6 beam. That should be glued and fastened with screws or nails in 2 rows with each fastener at 12 inches. The top row should stagger the lower row of fasteners. Meaning, if you start the top row at 12 inches from one end, start the lower row at 6 inches. And each row should be set about 1/3 the width from each edge. So the top about 1 and 7/8" from the top edge, the bottom row set the same from the bottom edge.
I don't think you'll need the extra 2x6 but since you've aleady did an awsome job on it, and if it'll help you sleep better, I'd say continue on and add it. It'll only make it stronger.
If you need more details, maybe an illustration, I know my explinations aren't the greatest, I'll try to draw what I explained out for you.
But this is lokking friggin awsome!
Can't wait to see the tank.
There will be a build coming... is there?
Well..
Get That Bad Boy Together and at the Least- Show Us Some Corals!!!;Woot