I'm going to be a weirdo and vote for an undefined tank leak mainly because I've dealt with tank leaks before, have designed my system so that most tank leaks happen in the garage sump, and looking for tank leaks is a small part of my daily tank observation (I am a lazy hard plumber, and accept the possibility of leaks due to not fully curing cement connections and/or making dodgy connections).
For example, two bigger leaks I have dealt with: exterior overflow cracked on arrival from BRS, and same overflow leaking from the overflow->lid connection. Instead of shipping the overflow back, I just attacked the crack with silicone and super glue, inside and out, and have not had a problem with it since. The leak from the overflow lid is just a fact of life I've learned to live with--water splashes inside the overflow lid, and through capillary action leaks UP and around the edge of the lid to brine out and dry on top of the lid; I can reduce it by reducing splashes inside the overflow, but short of replacing the overflow I see no way to permanently fix it (no water reaches the floor--it dries into salt lines running down overflow and/or tank). I've also had plumbing connections I've had to replace, bulkhead headaches, cracked sumps, and pretty much every other bonehead mistake that causes leaks.
Been fighting hair algae since April, and I'd trade nuisance tank problems for a plumbing problem any day of the week--plumbing/leak problems can be fixed in a day, usually with less than $5 in materials, and they RARELY kill or damage my corals.
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