I would not suggest using lard in a deep fryer. It has a 370 degree F smoke point temp. This means when you are deep frying your fish at 375 degrees, the oil will be smoking.

Here's some good infomation on the various types of oils and their best uses -- as well as the smoke point temp.
Smoking Points of Fats and Oils, Cooking Oil Smoke Points, Whats Cooking America