I have an Elite 4hdi and it does great in the daytime. At night it seems to pick up a lot of trash and is unusable. I don't know much about fish finders. Does anyone know what could be wrong?

Have you tried changing your sensitivity at night?

That "trash" may be plankton that suspends at night and returns to the bottom during daylight.

 

This is a early morning plankton column that was gone a few hours later:

 

change sensitivity and play with that setting up and down till you get what you like.  Post above is true about the trash the sonar is picking up.

have fun with it.