Backyard Aquaponics
A place to discuss aquaponics

I have a 400 gal Galvanized water trough that was used for cows, it is ok to raise tilapia in?
I see most systems are made from plastic or pvc.

Not unless you line it. Zinc in the galv is toxic to fish..

could I double line the tub with 6 mil polyethylene sheeting?

As in flexible pond liner, or a stiff sheet? You just need to be sure your water won't come in contact with the metal.

If I were you, instead I'd consider using a non toxic, potable water safe, water based bitumen paint. So much easier, and more robust longer term, and a better looking result than mucking around with polyethelene and all the associated joins/folds etc

eg http://www.awsproducts.com.au/crommelin-pondsealer

Costs us about $12 to 15 per litre here in oz, depending on what quantities you buy it in

I reckon around 2 litres would cover a tank of your size nicely