. . . they are made of plastic.

Tanks to catch rainwater from your roof are a great idea but if thoughtlessly placed, can massively increase the risk of subterrranean termite attack.

The tank should not be so close to the wall that you can’t see behind it.  A sight line and air gap of 150 mm (6 inches) is a good idea.  That makes it less likely that termites will be buidling hidden shelter tubes up the wall, and will allow you to spot them if they do.  If your tank must go against the wall (design constraints), then it needs to be raised on a platform that you can see behind and which allows for full and easy inspection..

The tank should be securely mounted on a firm surface.  If your house relies on termiticides (poisoned-soil) such as is applied with perimeter sprays or reticulation pipes, you must make sure that these are not defeated or damaged by the installation works.

Even if the tank is to be placed well away from the walls, you still need to be sure that excavations for new pipes have not provided an easy path for termites. Subterranean termites like to tunnel in the softer earth of pipe trenches.

The soil down below a tank will be at a more constant temperature and will tend to retain moisture.  This makes it a nicer place to be a termite.  If your tank sits right on the soil or sand, termites can come up beneath it searching for condensing moisture.  They’ll take a bite out of any soft materials they find. In some eastern States of the USA, tanks (and above ground pools) may occasionally be sat on an organic material, such as peat moss. This is not a great way to defeat termites. Much better to sit the tanks on a bead of fine crushed rock or coarse sand.

Be careful with where your overflows go. Once a tank is full, you want any excess water to  drain away far away from your house.  Water soaking in against walls or under a house is a prime factor driving termite infestation.

Oh, and make sure your tank’s input and output pipes are well screened so it doesn’t become a mosquito farm.