Introduction: How to Mix a Batch of Sea Salt(-water) for Your Marine Tank

In this instructable I will show you the right way on how to make a saltwater solution for your aquarium , without measurement tools.
Oh and if interested, the animals you see above are Aiptasia Anemones, these are so hardy, you can keep in a cookie jar.

Step 1: Which Salinity Do You Need?

So, look in the internet or ask in your pet shop, which salinity your animals need. There are two representations of how to describe the salinity, one is ppt (parts per thousand) and the other is the so called specific gravity (sg). To calculate the salt you will need, you will have to work with the the ppt scale, you can convert sg to ppt in an online calculator, for example this -->

Step 2: Learn What the Salinities Mean

This is the second lesson, here you learn what ppt means. If someone tells you for example that you need a salinity of 35ppt, he means there are 35 parts of salt in 1000 parts of salt and water mixture , so the relative water amount is --> 1000 parts total - 35ppt (the salt concentration you need) = 965 parts of water. So what does this finally mean? Well, not more, than you will have to mix 35g Salt in 965ml of water to get your final salt concentration of 35ppt.
You ask "was this all?", no surely not, you can't mix these two together and everything is fine, there are some important factors to calculate, for example the water impurities.

Step 3: Calculate the Water Impurities

So as you may have already suggested, this is the next step. If you look onto your sea salt package, you will read something like "each kilo of sea salt will give 30 litres of sea water at a density of 1.020", then you have to calculate, which amount of sea salt you have to mix in 1 litre of water. In my case, you will simply have to devide the amount of salt through 10, because (10 litres/10= 1 litre), now we know, that you need 33.3g of salt to get a salinity of 1.020 (27ppt at 20 centrigrade), but normally you should get 33.3ppt, right? You may be a bit confused after this step, but you can easily explain the strange difference in salinity. Well every salt is hygroscopic and so each brand of sea salt will have its very specific amount of water impurities inside, in this case the water impurity is 33.3ppt - 27ppt= 6.3g (per 35g of salt) -1 (thats approx. the amount of salts other than sodium chloride, you can use this as a constant in every salt mix) = 5.3g (per 35g of salt).

Step 4: Calculate Your Final Salt Amount

Now this is the last practical step. What do we need to know in this step: the salinity you want in ppt and the water impurity in your salt mix.
So how to calculate your final salt amount: (salinity in ppt) + (water impurity) = salt you need to add per thousand parts in total. Again confused? I will show you how to proceed in my example: 35ppt (you required salt amount) + 5,3g (water impurity) = 40.3g. Now do the same procedure as before in step 2 (or was it 3?) -> 1000 parts in total - parts of salt= amount of water you will need. That means in my case, I need 1000 - 40,3g = 959,7ml Water.

Summary: You will have to add 40.3g Salt to 959,7ml of water (don't know, how to measure? No problem, you can equate the amount of water in ml with its weight in grams)

post scriptum: This is not the exact way of calculating the amount of salt, but the differences are negligible if you work in salinity ranges between 27ppt and 40ppt, standard is 35ppt

Step 5: Hints:

I would want to add a few things:
1. never use pantry salt, always use sea salt or pet salt, because pantry salt contains iodine
2. Never use untreated tap water, because it contains chlorine, use whether reverse osmotic water, de ionized water, or dechlorinated tap water
3. If this instructable is too complicated for you, you can use an online calculator
4. always add salt to the water and not reversed, because else it may form lumps

Last but not least, thanks for reading my instructable and sorry for my bad English