Mixing salt (as a noob)

Well I just tried my hand at mixing salt for the first time, instant ocean with Store bought R/O water.
I leaned some things, firstly, a 5 gallon bucket literally holds 5 gallons (believe it or not), so if said person puts salt in the bucket first then mixing the 5 gallons is not an easy task as I found out.

Secondly there is still some salt crystals in the bottom of the bucket, not enough to be visible but I can feel them, is that a problem?

Ive mixed it how I stated and have it sat with a heater and an airstone in the bucket. I just took a measurement of the SG and it read 1.021. My tank reads at 1.018. Bear in mind I am completely new to this and got this setup today, I brought about 10 gallons of the original water with me and that's what's in the tank rigtht now, I'm mixing more to fill it up (20g long)
I know that the 1.018 is really low for the urchin and sea star, since the new water I just made is reading higher at 1.021 will that bring up the salinity some? Also how should I add the higher salinity water in safely as not to shock the critters inside?

Stock:
yellowtail blue damsel
green/blue chromis
scooter blenny
clown fish
cleaner shrimp (skunk I think)
urchin (I think it's a pencil)
sand sifting sea star
a couple of snail but don't know the variety
I saw 2 hermit crabs on the live rock as well as a regular looking crab

Tank is FOWLR, there's a ton of green hairy algae all over the live rock, the previous owner used tap water I believe so I'm hoping it dies away as I start using R/O.

 

1/2 gallon every 1/2 hr

Sent from my SAMSUNG-SGH-I337 using Fish Lore Aquarium Fish Forum mobile app

 

1/2 gallon every 1/2 hr

Sent from my SAMSUNG-SGH-I337 using Fish Lore Aquarium Fish Forum mobile app

Ok thank you, I already added the water since I wasn't getting any replies. But I did span it out and add a little every 15 or so minutes, so I'm glad I was headed in the right direction with my actions.

 

Would it be ok to mix a batch of saltwater and leave it for a week or two or does it need to be made closer to the water changes?

 

You should never add water to salt. You need to fill the container with water then start adding salt to it. You have crystals in the bucket because super saturated solution caused precipitation which you don't want. A good mix should not have crystals at the bottom.