Can you articulate why you need to cut the flow? If you calculated the filter for the tank size, I wouldn't cut the flow. It will not give enough water turnover to adequately clean and process waste.
Two things come to mind however. #1 What kind of filter do you have? A
canister filter would be a concern to me to mess with the flow, if it isn't designed to do so.
Most filters shouldn't have water cut on the intake side, this can cause motor problems.
If it is a bio-wheel , or other
HOB filter that floss would fit in, you can, but this COULD cause overflow if not watched.
I use a sponge on my intake for tank with fry or other small inverts. This may be a bit hard on the filter, I don't know, but it keeps things out of the filter, and the sponge adds to the biofiltration, and can be swished out to clean it.
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