Place the fillets in a bowl with the buttermilk. Stir to coat the fillets and allow them to marinate in the refrigerator for an hour.
In another bowl, blend the egg and soda water together with a whisk.
Pour the buttermilk off the fillets and replace with the egg/water mix. Chill in the refrigerator for 15 minutes then drain off the liquid.
In a medium skillet, over medium heat, add the butter and the canola oil.
Dredge a couple of the wet fillets in the bread crumbs and lay them into the hot oil. Don't crowd the fillets -- you may wish to fry them in two batches. (If you have to use a little more butter and oil, that's okay).
Fry each fillet 3-4 minutes per side, turning them one time only, until the coating is golden brown.
When the filets are done frying, lay them out on a couple of paper towels to drain and salt them right away. The thickest part of each fillet should be white and flakey inside.
Serve on sandwich buns with tartar sauce or, plated up with home made french fries.