Assuming your making more then one of the same part here are a few ideas without going into a lot of details. You can find more detail on these techniques by doing a search of this website.
You could vacuum form the parts but this require a vacuum form machine and a master mold, which can be made of clay, balsa wood or some other material.
If it’s a fairly simple shape you can build a master and support it on a “pedestal”, mount the styrene in a frame of shorts, heat it until it droops over a candle, stove or with another heat source, then pull it down over the master. With this and vacuum forming you may have to build the tank in a couple pieces and glue them together. I think someone posted pictures of this technique recently.
You could do a resin cast; I’m looking into this right now for a project I’m working on. This requires the casting materials and a master also. Did a web search and came up with a product called Alumilite, their website and a company called Freeman supplies has videos on casting parts.
For round items like tanks you might be able to wrap your styrene around a wood dowel craved into the shape of the tank, secure it with rubber bands and drunk it into warm/hot water to form it.
Be careful with whichever technique you decide to try because most require heat or chemicals.
The poster of this reply is not responsible for any injuries cause by trying these techniques. Results may vary.