Look for a caterpillar.

I can't tell from the pic but do you have deer problems too? Leaving droppings?

The biggest problems I have on lettuce is slugs.

Hmm.. there is no deer problems because this lettuce is living indoors in an aquaponic system. This means that water is constantly running along its roots. And deer droppings aren't that small... It might be a caterpillar... but I dont see it anywhere. Are there any type of water insects that might be a problem? It doesnt bother my strawberry plant that's growing right next to it. All I see are a ton of bite marks that appear every morning. I'll try to look for caterpillars by looking through the leaves..

Problem solved. It was a caterpillar.

The pictures keep uploading sideways... so i'm sorry if it's hard to see the picture very well. : /

Thank you soooo much for helping me out. : )

Caterpillars start taking on the coloration of what they've been eating, so they can be the devil to spot.

I missed where you said this was growing indoors in an aquaponic system but those still look like deer droppings to me and quite large too :(

Gumby, lol yes they do... Those are expanded clay pellets though..

Oh, I can now see why you thought they were deer droppings, Gumby.
You were probably referring to the clay pellets like what howelbama said haha.
I was referring to the black dots on the lettuce leaves. Not the brown ones.

But again, thank you sooo much to all of you who tried to help me out.
I knew there had to be something living in my lettuce.. but I didn't know what it was. I used to live in missouri and I never had any caterpillar problems before for some reason. But then I moved back to California and now I'm finding all sorts of pests I've never seen before. : )

Thing is the worm had to come from somewhere. Have moths flying around?

No, there were no moths. I do remember when I purchased my lettuce, there were caterpillar droppings on there to begin with. I didn't pay too much attention to it because I thought it was just dirt... or something. I rinsed all my lettuce with chlorine water very thoroughly... and I don't know how the caterpillar managed to stay on. I need to inspect my plants better before purchasing haha.