Q: Can I use ocean water (Natural Sea Water or NSR) in my aquarium?
A: If you have access to clean ocean water (NSR or Natural Sea Water) to put in your saltwater aquarium, by all means use it. Dissolved trace minerals and salinity may vary slightly by area, but since natural sea water is what marine animals live in, it can often be a better source than using fresh tap water and a commercial salt mix to make up solutions, which can be time consuming and costly.
Collecting natural sea water and using it in your saltwater aquarium has a number of advantages over using a sea salt mix and tap or even reverse osmosis water. A number of people have described the water in the oceans as a "soup" because of all of the "bottom of the food chain stuff" in it that makes the oceans work. Depending on where you collect your ocean water, you may be able to see the planktonic life (a wide variety of different alga, fish and invertebrate eggs, larvae, even small fishes and invertebrates) with the naked eye. Put a few drops of almost any water from the ocean on a slide and look at it through a microscope and you will be amazed at what you see. One of the advantages of using natural sea water in your tank is that you will also be getting the microscopic life in the ocean back into your tank. If you have corals, (particularly soft corals like Mushrooms) in your tank, they will appreciate the extra "stuff" in the sea water as they get a portion of their nutrition from absorbing the "stuff" through their surface as opposed to actively feeding.
When deciding to use ocean water, just be sure to collect it away from freshwater river and stream run-off areas, especially if they are near chemical plants, factories, and animal or agricultural farms where the water may contain toxins, or when fuels are present in places such as around boat docks and high traffic harbors.
Back when we were collecting marine tropical fish and invertebrates on Moloka'i in Hawaii, we got all of our water directly from the ocean. When we were out collecting, we filled the two 50 gallon barrels on the boat with sea water for holding the collected fish on the boat and then for transport back to our holding facilities. We used the same water for shipping the fish and invertebrates, after adding an ammonia buster, Amquel, to keep the ammonia produced by the fish in check and also an antibiotic to kill off any bacteria in the water.
After we had shipped out a load of fish and inverts, we would have to refill our 3,500 gallon holding system with new saltwater. If the water was clear at the boat launch in the harbor, would just back our pick-up truck into the water and fill the barrels in the back of the truck, getting what we needed there. If the water at the wharf was cloudy due to runoff from a recent rain storm, we would just launch our boat and motor out to where the water was clean and load water from there. We were fortunate that there were no large golf courses on the island which used chemical that would eventually leach back into the ocean and contaminate the water.
If you do not have access to ocean water, did you know that there are some public aquariums and water companies where you can buy filtered sea water? The prices that most of these outlets charge for sea water is really quite reasonable, especially when you take into account the price of a bag of sea salts and the time and energy required to mix up a barrel or even a bucket of saltwater.