How to Make a Sound

Pacific Science Center’s Puget Sound Salt Water Tide Pool is designed to look like a natural habitat, but behind the scenes it functions very much like any other aquarium. In this case, a salt water system.

Each week, we remove a portion of the water from our Puget Sound Tide Pool model and replace it with 100 gallons of new, clean saltwater. Because we don’t have ready access to Puget Sound water, we use a commercially purchased synthetic salt: brand name- Instant Ocean. Really! When this synthetic salt is mixed with the appropriate amount of water and other additives, it becomes chemically comparable to the water found in Puget Sound. In our tide pool, we aim for a salinity of about 3.2%.


First, we add a measured quantity of salt to our new mix water. We then mix the salt into the water overnight, using a pump dedicated to circulate the water. This also allows the water to release any chlorine as a gas that might be in the water.


Each time we do a full water change, we test the newly mixed salt water and the water in our exhibit using a hydrometer, which measures the specific gravity of the water. A hydrometer measures how dense a liquid is compared to water at a certain temperature. Because water can dissolve solids without changing volume, the more solids that are dissolved in water, the more dense the solution becomes.

In our case, nearly all the solids in our water are salts. So our hydrometer measurement is a pretty good indicator of how much salt is in the water.

Once we know the specific gravity of our tide pool water, we can convert to salinity – in fact our hydrometer converts for us. Salinity measures the parts per thousand of salts in the water. In our case, the water is about 3.2% salt – a bit less than the ocean because 11 rivers feed fresh water into Puget Sound estuary.

If the ambient temperature is very hot, we can lose water from the tide pool simply due to evaporation. When this happens, the water becomes too salty and we need to add less saline water to dilute it. Our system rarely experiences this. We do suffer from salt creep, a situation where salt crystallizes out of the water. If that happens in the pipes, it gives us quite a headache because the water circulation slows down.


Operating the tide pool, or any aquarium, is a great opportunity to brush up on math and chemistry lessons from high school. Math helps us calculate the amount of salt needed to bring a given volume of water to a certain salinity; chemistry helps us understand when and how to add buffer and how the nitrogen cycle allows wastes to be broken down. These skills are integral to maintaining a fully functional aquarium - just as important as knowing the names and life cycles of the animals we care for.