You don't want to overtighten, snug is good, tight is bad. Check your o-rings though, with good condition o-rings you only need it to be snug and that ring will fill in the space perfectly. If your o-ring is damaged though even overtightening may still leave gaps (and overtightening can even push the o-rings out of their intended slots and be worse than undertightening). So make sure those are all in good condition then tighten until it's snug, if that doesn't fix the issue... not sure, typically leaking is either a bum o-ring or your cotton has gone bad and needs to be replaced (in this situation a new coilhead). It's not a topbox but the old Kanger subtank, but the one my brother has just leaks, no visible issues, it just leaks after a while without being used. Personally turned me off of Kanger products.