I bought a used 350 last week: it passed the initial leak test, and I've just finished filling it up with fresh salt water, substrate, and aquascape. This morning, however, I noticed a leak had sprung at the base of the main return. I've resigned myself to emptying out the tank (fortunately, it does not have any inhabitants except two snails), but what should my next steps be at this point? I notice that the previous owner had sealed the site with an ugly black silicone. Remove and redo? Replace the o-ring? Any other suggestions?