Here's what I do and people who breed shrimp and fish do.
You can either buy an aquarium sponge (I've used the smallest aquaclear filter sponge) and cut a whole in it so that it fits snugly over the intake tube covering any openings.
Or they sell a pre filter at petsmart by fluval that does the exact same thing only it's pre cut, round, and black so it's a little more "stylish"