Sounds like there is some moisture fInding it's way in there. Is the container sitting in direct sunlight or an enclosed area which could be causing it to sweat?
no gasket on tropic marin as far as I have seen, but the lid is airtight. I know when I use it if I don't immediately wrap and tie off the bag and then seal the bucket lid it will clump. You have to imagine it is probably the most hygroscopic substance in the world and you are likely storing it next to a tank that is basically pumping water into the air. It won't hurt anything unless it really gets wet, but I've really reduced clumping by basically treating the bucket like it is a deepfreezer. The lid is only off for moments and I seal it before doing anything else.
I use Red Sea Coral Pro and its the same thing, it comes in a bag and therefore no gasket, so I use an old Instant Oceans tub and pour into it because it has a gasket. And the buckets a little bigger, so it makes for easier shaking around too.