It will warp, but if used for a tank, the sides and corners are sealed and bonded in place so warping isn't really as much a problem as when acrylic is used for tops.Regular acrylic will warp under any heat or any humidity regardless of what u use it for. U can use lexan/polycarb, but it gets expensive and is hard to work with. Glass is easiest to work with in my opinion.
Hope that helps a tad...not a great or detailed explanation, but hopefully it helps convey the gist.That is a very common question. The material is not defective, as many fabricators have claimed. That is the nature of the material. Acrylic is a hydroscopic material, that is to say, it has an affinity for water and it will absorb moisture into the microscopic pores in its surface. Normally, in the cast of clear acrylic without the mirror backing, it absorbs moisture evenly on the face side and the backside and releases it evenly depending on the relative humidity.
The bowing in the material occurs when you seal the backside with a non-porous coating, which prevents the normal absorption or release of moisture on the coated side. This produces an imbalance within the sheet, which may be exacerbated by conditions of high humidity during the summer months, or near coastal areas where humidity is highest.