No additives as of yet since there's no corals yet just a couple fish. I've never even seen this o-ring you speak of either. It wasn't there when I bought it and I never needed it before when I had it hooked up to an older tank. The collection cup always just sat in place and I didn't need to adjust anything. Maybe I'll pop by the LFS tomorrow and have a look at a new setup to see what I'm missing.
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Jason
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