Working on my morning coffee, so here are some random thoughts Roger...

You would be adding more water to the system, which helps for a more stable system.

I think gunk/solids will eventually build up in the bottom of the barrels, ceating an anarobic mess.

The heat your wanting to 'store' in the barrells, would be going into the entire system. Good in winter, bad in summer (depending where you live and the fish you raise)
Paint them black for winter, and wrap them with some reflective foil in the summer, if needed.

Why not add the barrels, and use them as 'stands' to hold some beds on top. Maybe 1/2 barrels like Alex mentioned, or mixing tubs ?
These barrels are used for making 'tea', but it may give you some ideas...