I had a custom made used 60g sump with no center brace. It was okay until I accidentally over filled the main tank and the sump got almost full. I say almost because the back panel burst, luckily I was in the room and shut off the main tank overflow and then started grabbing buckets and the shop vac. What a mess!
I replaced the back panel and added two 5" cross braces on top. I tested it full for leaks and have had no problems in over a year. I would just added a couple of new cross braces across the top and you should have no problem with bowing.
Note that the back panel did not have baffles across it. The sump was originally for a salt water tank and had live rock in the first half with no baffles (it was divided length wise by a glass divider) and had baffles in the second half for media. I use it for fw with lots of pot scrubbers and the occasional bag of carbon. If it had baffles across the full width, I doubt that it would have cracked. But top braces are cheap security!
I replaced the back panel and added two 5" cross braces on top. I tested it full for leaks and have had no problems in over a year. I would just added a couple of new cross braces across the top and you should have no problem with bowing.
Note that the back panel did not have baffles across it. The sump was originally for a salt water tank and had live rock in the first half with no baffles (it was divided length wise by a glass divider) and had baffles in the second half for media. I use it for fw with lots of pot scrubbers and the occasional bag of carbon. If it had baffles across the full width, I doubt that it would have cracked. But top braces are cheap security!