Peter Pan wrote:

Could I use something like Instant Ocean?

I asked the same question elsewhere when I needed to add salt to one of
my freshwater tanks in a hurry, only mine was Aqua Medic Reef Salt.
Although, the answer came back that I probably could in an emergency, on
balance I decided against it - cost being a major factor - a very
expensive way to salt a tank let alone a pond...oh, and also that I dug
out some "freshwater aquatic salt" that was hiding in a cupboard...

Go with something cheaper....it'll cost you a fortune to use Marine Salt
even if you do have it to hand

Gill