Should have a strainer,but if still sucking fish to the strainer it's better to use a foam sponge slitted and placed over the strainer to diffuse the suction over a broader area.
I use the foam blocks like used in AquaClear filters,make a slit in them and slide them over the intake strainer.
Nylon stocking can be used,but if the filter suction is strong it doesn't diffuse it and fish can still be sucked to it.