Have not had luck with silicone and such. Should not leak, some times it takes a day or two with a full load of water before it sorts it's self out. There is a product called Water Weld that works good.
I had to use a silicone type sealant to seal a few leaks I had on a 5 gallon bucket RDWC setup. DAP Aquarium Sealant was the particular brand that I used. Worked fine until I would start moving things around too aggressively.