The reason it goes is because it's not joinery grade timber. You can buy joinery grade timber and HAVE it treated and you're more likely to get consistent results. Best have a word with your local saw mill and ask if they do their own treatment (the one I once worked at still does and we often treated machined joinery sections) and ask their opinion.

Bear in mind that the joinery grade timber you buy from merchants has been kiln-dried and you're intending to put it into a large cylindrical tank, subject it to a vacuum to suck out any moisture still in it, fill the tank with water treated with chemicals, pressurise that to force the treatment into the pores of the wood, then reverse the process to remove excess moisture, which still leaves it wet. It's a toss-up as to whether your wood will take too kindly to that, but it won't distort (if at all) anywhere near as much as your common or garden p/t stuff.

Otherwise, look at some of the cheaper hardwoods, like sapele - a nice wood to work with and a pleasing colour.