Superglue or aquarium sealant would be your best bets. If you go for superglue, check he label to make sure it's a pure cyanoacrylate one (it will say on the packaging somewhere).
mine did exactly the same thing, I just laid a square of cling film over the open end of the spray bar tube and pushed the end cap in, the Clingfilm taking up any gap and trimmed any excess with a pair of scissors.
I can assure you that superglue is totally safe though; it's often used in reef tanks for securing coral frags and things like that, and marines are a lot more sensitive than freshwater.