Yes. It's important to cover your tank for many reasons, here are some:
- Trout jump, and will jump right out if you don't stop them
- Shading the tank stops algae growth - algae is a plant and hence a competitor to your veg.
- Birds and other predators can't take your fish
- Leaves and debris doesn't fall into your tank
- Evaporation is greatly reduced.
I recommend using shade cloth - just make sure you wash it in hot water first (no soap!) as most shade cloth has an anti-bacteria agent, and bacteria is essential in an aquaponics system!