I agree with addy1, if it's bowing out then it's not performing its primary function well, which is to hold the amount of water it says it holds. It might be defective, can you exchange it for another one where you purchased it from? If it's not defective and just cheaply made, building a frame or wall of some sort is really the only option if burying it isn't one. You can use pressure treated 4x4's, less expensive landscaping timbers, or some of those stacking landscaping blocks that you can use to build firepits, small retaining walls, or raised beds. Unless the frame is touching the sides, it's not going to be super supportive all the way around unless you put the tank in it empty, then backfill the gap with gravel. Be sure that if you use gravel that it's not sharp enough to poke through the tank (or add a layer of carpet between the tank and the gravel). Pea gravel is smooth, crushed stone is not. Pea gravel would work fine, crushed stone would need a padding layer between it and the tank. Water expands when it freezes, and if your tank is bowing just when filled with water, then it's also going to bow when it freezes, so good thick padding would probably be necessary if you're in a zone that freezes in winter. That being said, if you purchased the cheap stock tank because you're on a budget, then all these options negate having purchased the cheap tank, and you might just want to take it back and buy a better stock tank (Rubbermaid brand is good).
Some examples of raised container ponds with supporting enclosures: