Acrylic absorbs water and when emptied, if off gasses it. This can take a while, maybe 6 weeks or more. Some of the bow might come out but probably not much.
If you wanted to get the bow out with clamping, what you would do is take a long bar clamp (with rubber caps) and place it across the panel - such that the bar is parallel to the acrylic that is bowed. Then tighten it down, but not super tight, not yet. Then, stick a bunch of shims inbetween the center point of the bar clamp and the acrylic. What will happen is the shims will tend to push the bar clamp off the tank, so you have to balance out the number of shims and the tightness of the bar clamp. Now, the bar clamp will flex inward as you tighten it, pushing on the shims, and straightening out the acrylic. This is where you might start seeing the bar clamp slip outward as you tighten it.
The solution to that is to place another set of bar clamps from front to back, with one end of the clamp on the main bar clamp and the other end on the backside of the tank. This will hold the main bar clamp in place and prevent it from pushing out as you tighten it.
You should be able to set it up such that you can get the bowed panel back to straight. Now the trick is that you have to have this all in place and then flip the tank over so that you can bond on the brace and leave it there. So you might flip the tank over and set the bar clamps in place with the bonding edge hanging over the end of the work table. Then you clean the edge, clean the brace, and then lift the tank up and place it on the brace and you're ready to bond.
A couple things to take into consideration are general acrylic bonding procedures: set the work on MDF strips covered in camper shell foam to even out the inconsistencies of the acrylic and work table, use the pins method, clean the bonding area with denatured alcohol and blow with compressed air, use the pins method and blow the joint out with canned air prior to applying the solvent. Also you will have to prop up the rear joint up with a dummy piece of material to level out the joint.
If you are doing this to the front and back panels (you didn't specify this, maybe the tank has a built-in overflow chamber and the back panel isn't bowed?) then you will probably have to do this in 2 separate steps, unless you can rig 2 main bar clamps and 2 cross-clamps (which you can, if you need to)
The question still remains about the polished top edge though. the best bet is that you will need to scrape this down a bit using a razor blade clamped between 2 piece of wood so you have a clean acrylic edge to bond to, and that can be a bit tricky, but if you've got some skill you can make this work (and you don't need to take much off at all, but I would do it manually as a router would cut a flat edge that would then end up at an angle after you clamped out the bow)
Regarding time until you remove the clamp - acrylic will generally bond to where you can't move it very quickly, a matter of minutes. But it takes a lot longer to be really structurally sound, and in this case where you are trying to remove a bow, I would leave it for a week. Not kidding, the longer you leave it, the better the bond strength. Also I would probably use a 1.5" or 2" wide strip of 3/8" not 1/4", that will hold up a lot better.
This is just what I would do, feel free to ask any questions if the above doesn't make sense.
As for ATM, they would probably charge you less to make a new tank than to fix it. And don't take a torch to it and try to heat the material up to take the bow out, heat is the enemy and it will ruin the acrylic. It will start to bubble and melt before you can take the bow out of it with heat. JMO (but from experience)
