"Why is there a different top to your Biotank 6 and larger tank sizes?"

The reason is quite simple.  Our Biotank 6 units are manufactured by a process known as Rotational Moulding.  We have a series of large moulds into which we pour powdered PVC.  This is then heated and rotated over what is effectively a large fire.  The plastic forms on the inside of the mould to create the tanks.  These are then assembled by hand to produce the final product.  The tank is then coated in insulation.

The Biotank 12 and larger sizes are made by fabricating Polypropylene sheets into a tank.  A much more laborious process as each part of the tank has to be cut and welded into position.  This produces a very strong tank that can be made exactly to order.  If there are specific requirements for a tank these can be accomadated into the larger tanks at the design stage.

The lids of the tanks show the differences.  Please see:

http://www.biotank.co.uk/biotank_installation_photographs.htm

These photographs show a number of different installations and methods of installing the tanks.