question on instruction

Mixing
The volume of salt water that can be made from a single bucket of salt will depend on the final SG that the salt solution is mixed to.

For a reef aquarium we suggest mixing to an S.G: of 1.025 @ 25°C = 35.5 ppt by using the salt to water ratio given below.

Using a clean container

Add 1 Kg of H2Ocean+ to 25 litres of water ( 1 Lb to 3.0 U.S. Gallons )
Mix well until all of the salt is dissolved and the solution is clear.
We recommend that you aerate for 24 hours and bring to temperature before use.
Check the S.G. before addition to the aquarium.
If required add more salt or water to obtain the correct S.G. reading.
Remember that S.G. changes with temperature.

What does that part mean? The one highlighted in red. I always used oceanic salt, and it would dissolve within couple minute. I don't understand that part.
Help please.

H2Ocean Natural Reef Salt | The Aquarium Solution

 

They recommend 24 hours so it has time to reach it's proper ionic balance as well as proper Oxygen saturation. Freshly mixed SW could be O2 deficient and could also sting the gills of fish. Doesn't mean it will, but it is possible, so that's why they recommend waiting 24 hours.

 

Yep, and you should also oxygenate the water for 24hrs prior to mixing the salt. My water changes are a 48hr process

 

no need to do that if your using RO/DI water as you should be, used to do that whith my freshwater tanks to disapate clorine without using another chemical to nutralize it but most municpalities have now switched to using cloramines which dont disipate nearly that fast.