i know it wouldnt hurt the fish because ive researched it before for another joint on a gravel cleaner.
but as you say, it breaks down it may cause damage to the filter.
i do think the way i would do it would stop the leak. but on the thought of a break down and it messing up my filter, its not worth it.
i have ordered a spare oring incase of an emergency with some food grade silicone grease. just got to get new gravel and a heater and then the cycling of my new tank will begin.
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