I didn't realize how much this board was twisted until I finished gluing the frame together (the clamp obscured it a bit). This would be where the top layer of plywood will sit.
Is it safe to use or best to replace? The frame isn't yet secured to the rest of the stand at least.
Place the plywood on top and see if it lays level, if not replace if it does it's your choice and asthetic. I would personally anyway bc I'm a little perfectionist minded but if it's overall straight it shouldn't bother structural stability.
The weight limit hinges on the legs not so much that board there. As long as your tank is level, should be good. I’d add an extra screw to the side right where it’s angled out at the corner but agree with others-it doesn’t HAVE to be replaced if it’s going to be hidden.
If you have fitting issues with your top or on the side of the frame, just take the screws out and use a hand plane to knock it down to fit. Most handymen have a #4 jack lying somewhere in the garage collecting dust, if you don't have one, maybe just borrow one from someone else.
What you can also do is flip the board around so that the bow is on the other side. May not make a huge difference but in my experience it is easier to force bend boards with the hollow bit in the middle together than the gap being on one side or both (hope that makes sense).
I would replace it. Its a small price to pay now for it. Tank is not setup, easy access, simple correction, than in say 3 years, full tank, a lot of money floating in the tank, moisture, complicated to remove and replace.
Take the time now, as you will not have the time and ease in the future.
It is the foundation of your tank, everything is built from this