You got plenty of room to put a bigger tank. If the trailer were mine I would find the largest tank that I can fit between the i-beams. I would take the trailer to a welding shop and have them cut out the tank supports, remove the tank and weld in new supports between the I-beams to suit the new tank.
I wouldn't worry about a tank that hangs from flanges like you have if I replaced the tank. I would have them put in better braces than the ones that you have so that fully support the tank belly so it won't sag when it's full of water. The supports you have right now cause a lot of stress on that tank belly it looks like it barely makes contact in the middle of the braces when the tank belly sags.
I'd also add some black coroplast on all sides of the tank to help keep algae and mold from growing in the tank. Mine has it installed from the factory.