There are two general types of water for your saltwater or reef aquarium: real seawater that you buy at the fish store or get from the ocean yourself and artificial saltwater that you buy pre-mixed from your local store or in powder form (artificial sea salts) and you mix into saltwater yourself.
We’re going to talk about how to mix up your own saltwater. It is easy to do but you need a few items and to plan ahead a little. To begin you’ll need to get the following – a high quality sea salt mix, a clean bucket or trash can (one that has never been cleaned with soap or used for anything else), a hydrometer or refractometer and a source of fresh water. A small submersible pump also comes in handy to help speed the mixing. If you are going to be using tapwater you’ll also need a product that gets rid of chlorine and chloramines (like DrTim’s Aquatics AquaCleanse).
Let’s start with your water source. Tapwater is ok to use for most saltwater fish tanks but if you are serious about your reef tank, you should consider upgrading to a DI/RO water purification system (another topic) so you can start with pure freshwater. If your tank is small, you can buy distilled water at your local grocery store and use that. Is it absolutely necessary to purify your tapwater? The answer is technically no, as some parts of the United States have very good tapwater, but it is recommended, especially if you are setting up a reef tank.
Once you’re decided upon your water source and have bought your sea salts, you can start mixing. But first you need to do a little math to determine the amount of sea salt you’ll need. Unfortunately, there is no standard amongst sea salt manufacturers as to how many pounds of sea salt to add to a given volume of water; every brand is a little different. Follow the instructions on the brand of sea salt you bought in terms of pounds of salt to add per gallon or 5 gallons of water to get the right salinity.
Speaking of salinity – this is a general term for the saltiness of the water. Seawater, and to a certain extent, artificial sea salts, are a complex mixture of lots of elements – both positive charged ones (called cations) and negative charged ones (called anions). You can read a detailed description of sea salts here. Technically, there are two ways hobbyists measure the saltiness of their saltwater: specific gravity and salinity. Specific gravity is measured with a hydrometer while a refractometer is used to measure salinity. I’ll talk more about these in another entry.
Ok so let’s make some saltwater. I do not recommend mixing sea salt in your aquarium. The salt does not dissolve quickly and it takes a lot of checking and time figuring out if the salinity is correct. If your tank is completely empty then it’s ok to mix in the tank; otherwise use a bucket.
Fill your bucket with a known volume water. Next add a measured amount of sea salt. You can add 3 to 4 times more sea salt to the water in the bucket than the recommend final amount. The more sea salt you add, the longer it will take to dissolve but the fewer buckets of mixed salt you’ll have to make. Once you’ve done it a few times you’ll have an idea of what works best for you.
Here’s an example – say you have a 30 gallon tank and your sea salt manufacturer recommends 1.5 pounds of sea salt per 5 gallons of water to reach a salinity of 1.022. The math is 30 gallons divided by 5 gallons times 1.5 pounds of sea salts. This equals 6 times 1.5 or 9 pounds of sea salts. Now in reality, you’re not going to need a full 30 gallons of water because the live rock and coral substrate and other things in the tank will displace some of the water, so start with less than the amount calculated. In this case say 7 pounds of salt. Let’s makes two buckets full of saltwater, so add 3.5 pounds of sea salts to the water already in the 5 gallon bucket. You can mix it by hand stirring with a clean piece of PVC pipe or you can also add a powerhead or small pump to the bucket to mix the water. Once the water is clear, pour it into the tank and repeat the procedure. In the meantime, if your tank has deadrock (versus liverock) you can add say 10 gallons of freshwater to the tank. Once the first 5-gallon bucket of saltwater is mixed, pour it into the tank and mix well. Now make a second bucket like the first one, add it to the tank and mix well. Now you have about 20 gallons of water in your tank that should be at least 80% full, if not more. Get the filter and pumps running and fill the tank with freshwater to the final level you want, and then let the water mix for an hour or so. Next, measure the salinity. If the water is too salty, you need to remove tank water and add more freshwater. If the salinity is too low, remove 5 gallons of tank water into the 5 gallon bucket and add some sea salts. How much sea salt to add depends on your individual situation; you just have to estimate and re-measure. But remember there is no one absolute correct salinity value – you’re trying to get into a range. For most saltwater and reef aquaria at 76 to 80 deg F, the range is 1.020 to 1.024 on your hydrometer. Don’t fret about getting it exact – there is no exact!
If your tank has liverock in it instead of deadrock, then you don’t pre-fill it with freshwater. Instead, add the first bucket of saltwater that you make and quickly add some freshwater and get a pump or powerhead going to mix the water. Prepare the second bucket of saltwater, add it to the tank and fill the remaining way with freshwater. Then continue like above, checking and adjusting the salinity to get between 1.020 and 1.024.
That’s it! Most hobbyists keep a bucket of saltwater mixed and ready to go with a little aeration. And don’t forget the water changes.
Lastly, you’ll notice that over time the water level in the tank will drop. Water is evaporating but only water is evaporating – not the sea salts. So you top up the water level with freshwater – not saltwater – or the salinity will get too high. Also you should let newly made saltwater circulate for several hours before adding fish or corals to it.