I had a sample of the water from the aquarium tested at a fish store last weekend.  I was feeling pretty confident when I went in to the store.  I’ve tested a number of chemicals every week, and everything was looking great.  But, I wound up learning an important lesson about calcium.  So, I figure this is a good opportunity to review the chemicals in a saltwater aquarium.  This post is likely to get a wee bit technical, but it’s important information to maintain a safe, healthy environment for your fish and marine invertebrates.

Salinity

A common hydrometer measuring the specific gravity of salt water

Salt water, of course, requires salt. Checking the salt content of your water is one of the most critical tests you’ll do.  Most aquarists use a hydrometer to test the salinity of their aquarium water.  It’s important to understand that a hydrometer doesn’t actually test the amount of salt that’s in water.  Instead, it tests the specific gravity of a liquid.  The specific gravity, also known as relative density, changes by a measurable amount as salt is added.  This property makes a hydrometer a good, inexpensive test.

The average salinity of the oceans is 1.0265 g/cm³.  This is also a good target for marine aquariums.  Years ago, aquarists believed that keeping aquarium water salinity lower than the oceans’ would reduce the stress to the fish in the aquarium, but the sources I’m reading and talking to now recommend a specific gravity target of 1.0265.

Now, if you look carefully at the hydrometer I’ve pictured on the right, you’ll notice it is registering a specific gravity of 1.022.  Yes, that is my aquarium water, and yes that is markedly lower than desired.  However, the fish store checked my aquarium water’s salinity, and they found that it registered 1.025 using a refractometer.  A refractometer measures refraction as light travels through a small sample of water.  Refractometers are more accurate and less susceptible to temperature changes and outside atmospheric pressure than a hydrometer is.  I’ve concluded that my hydrometer is measuring a little bit too low.  As a result, I’ll be continuing to use 1.022 as my salinity target as long as I’m using the old hydrometer.

Remember to replace any evaporation from your saltwater aquarium with fresh (RO/DI) water.  This is because the salt does not evaporate.  If you don’t replace evaporated water with fresh water, your salinity will gradually rise.  If you replace evaporated water with saltwater, salinity will rise even faster!

Nitrogen Cycle

Another group of critical tests is related to three  nitrogen based chemicals.  These chemicals are referred to as the nitrogen cycle because different algae and bacteria in the aquarium consume these chemicals, and release them as other chemicals in the cycle.  The process of breaking down these chemicals is known as bio filtration.

The first stage of the nitrogen cycle is ammonia (NH₃ / NH₄⁺).  If you’ve smelled strong ammonia based household cleaners, you can understand why this would be a bad chemical to let build up in your aquarium.  Ammonia is poisonous, and can cause lots of problems for your pets.  Fish and many marine invertebrates generate ammonia as waste.  Also, rotting food and dead animals will create ammonia.  In a relatively small environment like an aquarium, a little bit of ammonia can be a big problem.

In a fully cycled aquarium, helpful bacteria, called Nitrosomonas, will convert ammonia to nitrite.  Once these bacteria are established, your ammonia levels should go to zero.  When ever you add new live rock or animals you should watch your ammonia levels, however.  The levels could spike as the Nitrosomonas adjust to the higher bio load.

Nitrite (NO₂) is the second stage in the nitrogen cycle.  Like ammonia, it is poisonous to fish and most marine invertebrates.  Also like ammonia, nitrite has its own bacteria, called Nitrobacter.  As the Nitrobacter consumes nitrite, it produces nitrate.  In a fully cycled aquarium, nitrite should also stay at zero, but can fluctuate when you introduce new live rock or animals to the system.

The third stage of the nitrogen cycle is nitrate (NO₃).  Nitrate can become a problem if it is allowed to build up in an aquarium, but it is not nearly as harmful as ammonia or nitrite.  Your aquarium inhabitants should be fine as long as the nitrate level is not allowed to go over 20 ppm.  The major problem with nitrate is that green and red algae love nitrate!  If your nitrate level goes up, you can expect an algae bloom to follow shortly.

In my original fish only aquarium, this third stage was as far as the nitrogen cycle went.  When the nitrate level got too high, I had to do a 10% water change.  This allowed me to remove a small part of nitrate rich water with fresh water that had no nitrate.  To keep up on nitrate levels, I had to do a 10% water change, usually once a week.

Luckily for me, the reef aquarium has something in it that has dropped my nitrate levels to zero as well: coralline algae.  Coralline algae consumes nitrate, and produces nitrogen gas (N₂).  The nitrogen gas just harmlessly bubbles out of the aquarium and into the general atmosphere.