Cylindrical sponge with a hole through the centre designed for some brands of internal power filter. They look like the sponge in the link Aiken provided and you slide them over the intake strainer of the filter.
-----------------
Most healthy fish can stay away from the intake of a power filter. If your fish are being sucked onto the intake strainer, either the filter is too strong or the fish are sick.
What sort of filter is on the tank?
Can you adjust the flow on the filter to reduce the current in the tank?
What are the tank dimensions (length x width x height)?