Most boats have either a surface mount or recessed mount vent with screens, like the picture. When they plug, and they always plug, I just take a 1/8" drill bit and drill the shiny stainless face of the unit. Too small for most bugs, big enough for air.View attachment 42878
Those are actually fuel vent thru-hulls...boat builders use 'em on every vent line--fuel, water and waste--because they can buy the same ones in bulk cheaper than they can buy the best ones for each use.
They're ok for water tanks, but not for waste tanks 'cuz they don't allow enough air exchange via the vent. I'd replace it with an open bulkhead thru-hull that will allow you to back flush the vent line every time you wash the boat. The occasional bug that manages to get into it will be washed into the tank....and unlike your fresh water supply, a bug or two can't contaminate sewage. They only plug up the vent line if you neglect back flushing it..
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