I have two identical 10000 ltr tanks feeding my factory. I need to balance the water usage from the two tanks as at present one tank fills and empties and the other remains about 3/4 full all the time. What is the correct way of plumbing these tanks as I have a film of biological s floating about in the second tank as the water does not circulate.

You have not indicated very clearly how the tanks are plumbed. Could you empty both tanks from the bottom and have the piping joined with a tee fittng and discharging out? Are the tanks vented to the atmosphere? Is one tank a different size than the other?