How to mix saltwater:

A note for newbies: Table salt must not be used to prepare salt water for your crabs. It contains synthetic (man-made) iodine which is toxic to hermit crabs. I would also advise not to use food-grade sea salt because it has been refined and not enough is known at this time as to whether or not it is safe. Natural iodine found in sea water and seaweed are essential to the overall health of your crabs and is necessary for successful molts.
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Cystal Sea Marinemix Bioassy Formula
 The site from which I purchased this mix indicates 1/4 lb or 1/3 cup will yield 1 gallon. I found it to only be 1.016 SG when mixed this way. I tested it using 2 different hydrometers-one that was also purchased at the same site as the salt mix. I mixed it using 1/2 cup and got 1.021. (I add an additional tablespoon for my C. Perlatus to raise it 1.023)
I did find that this took longer to dissolve than Oceanic.
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Doc Wellfish's Aquarium Salt
package indicates for brine shrimp hatching to use 8 tablespoons for each gallon (or 1/2 cup)

When I used DW, I added my de-chlorinator drops then a small amount of extremely hot tap water, then my salt crystals. Cap the bottle and shake vigorously to help the crystals dissolve. I usually ended up waiting 30 minutes to an hour for them to dissolve completely then added the remaining water. I switched to Oceanic simply for the ease of mixing. My crabs still get the crystals in their food dish
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Instant Ocean
package indicates to use 1/2 cup (which=8 TBSP) per gallon.

*reports have shown that IO has some of the highest metal concentrations..something to think about. Check it out here http://www.reefkeeping.com/issues/2003-03/rs/feature/
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Oceanic Natural Sea Salt Mix
package to make 5 gallons indicates .29 lbs/gallon. I measured the package and it was approximately 2 1/2 cups. Which is 1/2 cup per gallon or 8 TBSP

*This is the salt I am currently using and 8 TBSP makes a specific gravity of 1.021. I use an additional tablespoon to raise the SG for my C. Perlatus. I also provide Doc Wellfish crystals in the food dish as well.
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Red Sea Salt
package directions indicate 2.8 lbs. dissolved in 10 US gallons, which is .28 lbs for 1 lb which again is basically the same as Oceanic...1/2 cup per gallon (or 8 TBSP)

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Tropic Marin Sea Salt
package instructions indicate 151 grams = 1 gallon. 151gms x .0353 = 5.33 ounces. 5.33oz/8(oz in a cup) = .66 or 2/3 cup per gallon. There are 31.68 or 32 tsp in 2/3 of a cup.

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ZooMed Hermit Crab Salt Water Conditioner Part II
package indicates 1 teaspoon per 8 ounces of water

That would get expensive...considering there are only 2.25 fl oz (or 13.5 tsp) in one of the small bottles-not even enough to mix a gallon. (1 liquid oz = 6 teaspoons)
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So basically all of the dry salt mixes tend to be 1/2 cup or 8 TBSP per gallon of water. If you are unsure if your sea water is "salty" enough or too "salty" you can purchase a hydrometer which measures specific gravity. The specific gravity of sea water varies depending on location, but is generally somewhere between 1.020 and 1.025. To get an accurate result with your hygrometer, the water should be around 77 degress farenheit.

If you are offering both a de-chlorinated fresh water dish as well as a de-chlorinated salt water dish, you needn't worry if the mix is too salty. The crabs can adjust their shell water accordingly. If you have C. Perlatus, saltier water is more beneficial as their shell water has been shown to be much higher in salt content than those of other species.

In addition, if the salt water dish is not salty enough, they will need to obtain natural sea salt from other sources such as their diet. One option is to provide a small pile of the Doc Wellfish salt crystals with their food.

Here is some additional reading about specific gravity (versus salinity)
Seahorse nw
Algone

................................FYI CONVERSION CHART.......................................


Thanks to Wendy at Hermit Crabs R Us for this list!!

Here is an article comparing many popular brands of salt: Interesting