This page contains affiliate links.  All that means for you is that if you click on one of my links and make a purchase, I receive a small commission.  This helps me keep this content 100% free!  But rest assured, I only recommend products I know and trust! Happy Reefing!

Saltwater Aquariums Demystified | Part 04: Water Chemistry, RODI, & Salt

Table of Contents

I) Introduction

Follow & Subscribe!

II) Water Chemistry

1) Nitrogen Cycle

colorful infographic of how the nitrogen cycle works
© Marine Depot

In an aquarium, the nitrogen cycle refers to various stages involved with turning ammonia into nitrate. Hobbyists will often say “My tank is cycled”, which means that beneficial bacteria have colonized and decomposing waste is now successfully being transformed from toxic ammonia to nitrates. In a saltwater aquarium, this process usually takes several weeks.

a) How To Cycle

There are three different ways to cycle a tank.

  • Using Shrimp/Fish Food– This method is the longest, as it will take up to 6 weeks.  All you do is add a piece of shrimp or a spoonful of fish food, remove your filtration, and let it decompose. Test regularly.
  • Fish Method– Purchase a hardy species of fish such as a clownfish or damsel fish.  Feed them regularly and the tank will cycle.  This can be stressful for your fish as they will be swimming around in ammonia for a while.  I don’t recommend this method.
  • Dr. Tim’s Fishless Cycling Method– This is my recommendation, as it does not put fish in danger, and it only takes from 10-14 days to cycle.  You will need to buy Dr. Tim’s Ammonia Chloride and One and Only.  Follow the directions carefully, and your tank should be cycled within 2 weeks.

b) Knowing When The Cycle Is Complete

You will need to test for ammonia, nitrite, and nitrate to determine when the cycle is complete.  First the ammonia will spike, then the nitrite will spike, and finally the nitrate will spike.  Once the ammonia and nitrite return to zero, your cycle is complete.

2) Water Parameters

When hobbyists refer to “water parameters”, we are referring to various measurements such as pH, temperature, salinityammonianitritenitrate, phosphatecalciumalkalinity, etc. Often when trying to get to the root of a problem, the first question a fellow hobbyist will ask is “what are your parameters?”

a) Basic Water Parameters

This is an arbitrary category, but I think it fits.  There are certain water parameters that you will want to be familiar with and track regardless of livestock.  These are:

b) Coral Water Parameters

These are the elements that your inverts need in order to build their calcium carbonate skeletons and shells:

c) Major/Minor/Trace Elements

  • Major Elements– Six in total, these make up more than 99% of the total composition of seawater. Elements with a concentration > 100ppm in seawater.
  • Minor Elements– Six in total, concentration of 1-100 ppm.
  • Trace Elements– I could not get a straight figure, but the best number I found was 82, all of which have a concentration of less than 1ppm in seawater.  Even though these are minuscule amounts, some of these are absolutely essential for life.

d) Don't Chase Numbers

When I was a beginner, I would obsess over every water parameter.  I lived in this dank apartment, and my pH would always hover around 7.8.  I had read that the correct parameter was closer to 8.2, so I set about trying to raise my pH.  I tried Kalkwasser dosing, leaving the windows open to get rid of the CO2, and even ran airline tubing to my protein skimmer.  Nothing helped.

But this is what I realized.  My pH didn’t matter.  It was fine.  The more important factor is to  keep things stable.

So my biggest advice for beginners is don’t obsess over your water parameters.  Track them, make sure they are stable, and address issues as they arise… don’t create issues yourself!

e) Ideal Water Parameters

3) Test Kits

To get started in this hobby, all you need is one of the basic test kits below.  That’s because you only need to test for ammonia, nitrite, and nitrate to determine when the cycle is complete. But as you add invertebrates and your tank ages, you will likely want to test for other parameters, so you’ll need more test kits.

a) Basic Test Kits

Red Sea Marine Care Test Kit

The most affordable is the API Saltwater Master Test Kit.  I own it, it works great… But, if you are willing to spend twice as much, you could buy the Red Sea Marine Care Test Kit.  It is my preferred kit, just because it is a bit easier to read (in my opinion of course!).

b) Colorimeters

I never recommend these to beginners, just because they are expensive.  But, they are super accurate and awesome.  I’ve slowly purchased these over time when the budget has allowed.  Instead of having to interpret the results based on your own eyesight, these colorimeters read the color for you.

c) Refractometers/ Hygrometers/ Salinity Probes

Buy the refractometer.  It is affordable and accurate.  But, sometime down the line you might opt for the ease of the Hanna Salinity Tester.  The Hanna Salinity Tester will need to be calibrated once a month, and the calibration pouches cost $1 (as of the writing of this blog).

4) Controllers

Purchase the Bayite temperature controller on the left.  You can plug in your heater and a fan, and set the controller to keep your aquarium water at a steady 78 degrees.  I own three of these, and I love them.  I’ve tried several different brands, but this one from Bayite is my favorite.  The temperature probe is detachable and replaceable, and the Bayite is super affordable.  Personally, I wouldn’t run a saltwater aquarium without a temperature controller, because if you just rely on the heater itself, you will get wide temperature swings.  

The Neptune Apex on the right is an advanced controller that I never recommend for beginners.  Don’t get me wrong, it is an amazing product.  But at $800, it’s not at all necessary and it can over-complicate things for beginners. At some point down the line, once you have more experience under your belt, you may decide to pick up an advanced controller like the Neptune Apex.  I have owned it in the past, but I currently run 5 saltwater tanks in my home, and I only use the Bayite controller.

III) RO/DI Filtration

a) RO/DI Basics

MD Kleanwater 4-Stage Advanced

Reverse Osmosis / De-Ionization Filter. RO/DI filters use various stages and types of filtration to achieve 0 TDS water. Water first passes through a sediment filter, then a carbon filter, then the RO membrane, and finally through de-ionization resin. Considered the gold standard for creating clean water for your saltwater aquarium, the various types of media do have to be replaced depending on the amount of water you make and the makeup of your local water supply.

b) Things To Consider Before Purchasing a RO/DI Filter

Just to make sure you get the right RO/DI filter for your needs, consider the following:

  • Stages– Even though RO/DI filters contain the same stages, some can come with duplicates to increase filtration power.  It is most common to find filters with 4-7 stages.  For example, if you live in an area that does not use chloramines, and is light on the chlorine, you probably only need one DI resin container.  But if you live in Southern California like I do, then two DI resin containers are probably best.
  • TDS Meters– You will need to purchase some sort of TDS meter.  It is the only way of ensuring your filter is working correctly.  It will also alert you to when it is time to change out your filtration media.  You can either buy a handheld TDS meter or an inline TDS meter.  An inline meter will give you up to the second readings during the filtration process.
  • Water Pressure– In order to push water through the RO membrane, you need to have good water pressure, somewhere between 45-85 PSI.  If your faucet/tap does not have good water pressure, you will need to purchase a booster pump to increase the pressure.
Booster Pump


  • GPD– The standard 4-stage RO/DI filter will produce about 90-100 gallons per day, depending on your water pressure.  If you need more output, you can purchase a second RO membrane and container, rework your plumbing, and you will nearly double the output.

c) Maintenance

Maintaining your RO/DI filter is quite simple.  If your system comes with an RO flush valve, merely turn the knob to flush out your system for 30 seconds before and after use.  That will increase the life of your RO membrane.

Other than that, you will need to replace the filtration media from time to time:

  • Sediment Filter– Once it starts turning brown, replace it.
  • Carbon Filter– Replace very few months or if your TDS starts creeping up.
  • RO membrane– These typically last between 2-3 years, but again replace if your TDS starts to climb.
  • DI Resin– I highly recommend using the color changing resin.  The DI resin cartridge will start to change color from the bottom to top.  Once the color change reaches to about an inch from the top, replace it.
RO Membrane Flush Valve, © My First Fish Tank

d) Auto Top Off Systems

Your saltwater tank will evaporate freshwater every day.  You will need to replace this daily with RO/DI water to ensure your return pump does not run dry, and to keep the salinity stable.  You will either have to do this by hand, or you can purchase an ATO system to do it for you.

There are many brands of ATO units, but they all come with similar feature:

  • Sensor/float switch to detect when the water level is low.
  • A pump to replace evaporated water.

You will need to purchase some sort of RO/DI water reservoir, such as a 5-gallon bucket.  Then, instead of having to manually top off your tank daily, you will only have to refill the reservoir when it is running low.

Reef Breeders Exo-ATO
Tunze Osmolator

e) Common RO/DI Questions

  • Can I Use Tap Water Conditioner Instead Of RO/DI Water? Simply put, no, you cannot.  Tap water conditioner detoxifies heavy metals and removes chlorine and chloramines, but it does nothing to filter out minerals or other dissolved solids.
  • Can I Use Distilled Water Instead Of RO/DI Water?  The short answer is yes.  Although many commercially available brands of distilled water do not come with 0 TDS.  Lab grade distilled water certainly contains 0 TDS, but in a recent test I did, the distilled water off the shelf was testing at 6ppm TDS. That’s probably not a terrible number, but 0 TDS is certainly preferred.

f) Our RO/DI & ATO Picks

MD Kleanwater 4-Stage Advanced

At the time of writing, the MD Kleanwater 4-Stage Advanced RO/DI filter was just under $200.  With an average production of 100 GPD, this will be more than adequate for most beginners.  The advanced filter comes with a water pressure gauge, and two inline TDS meters.  If you place the first TDS meter after the RO membrane, and the second TDS meter in the clean water output, you will have a really good sense of when it is time to change out your filters.

Reef Breeders Exo-ATO
Tunze Osmolator

These are the two ATO units that I currently use.  I currently run two of the Reef Breeders Exo-ATOs, and they work great.  They have a single optical sensor, and are under $100.

The Tunze Osmolator is my favorite ATO on the more expensive side.  Coming in at around $200, I’ve owned mine for over five years, and it has never failed me once.  It comes with both an optical sensor, and a backup float switch in case of malfunction.

IV) Salt Mixes

a) Where to Get Saltwater

Alright, there are three places to get your saltwater from:

  • Ocean– there companies out there that sell seawater.  Do not just go to the beach and collect it.  You don’t know if it’s legal or if its clean.
  • LFS– Your local fish store will likely sell you pre-made saltwater for around $1.00/gallon.
  • Make It Yourself– Probably the easiest and cheapest in the long run, you’ll need a source of clean water (RO/DI or distilled), and commercial salt mix.

b) Differences Between Commercially Available Salt Mixes

There are well over 10 commercially available salt mixes on the market… and they will all work just fine.  But they all do have slight differences.  Some have higher alkalinity, and others are made from pharmaceutical grade ingredients.  Some are meant for FOWLR systems, while others are designed for SPS dominant tanks. I’ll give you my recommendation below, but here is a selection of salt mixes that I’ve used.

c) How To Mix Saltwater

At the heart of it, this is very simple.  If you are filling up your tank for the first time and there is no livestock present, here’s all you have to do:

  • Get a bucket or food-grade trash can and add RO/DI water.
  • While stirring, add scoops of salt mix until you reach a specific gravity of 1.026.
  • Then carefully add it to your tank.

If you already have livestock and you are doing a water change, the only difference is you’ll need to heat the water up first so it matches closely the temperature of your display tank.

You can automate this entire process a bit by adding a heater and utility pump to your mixing container.

Brute 20 Gallon Food Grade Trash Can With Lid
Brute 20 Gallon Trash Can
Maxi-Jet 1200 Powerhead

d) Our Recommendations

There will be hobbyists who disagree with this recommendation, but here is why I choose the Red Sea Coral Pro Salt Mix:

  • Especially for beginners, the higher calcium, alkalinity, and magnesium content means that, as long as you keep up with your water changes, you won’t have to worry about two-part dosing unless you heavily stock your tank with inverts like corals and snails.

For measuring the specific gravity/salinity of your seawater mix, a refractometer is your best bet.  It is the perfect balance of affordable and accurate.

V) Additives

I’ve broken additives down into four categories, even though I know that there are some outliers!

a) Two-Part Dosing

Corals build a calcium carbonate skeleton, and snails build a calcium carbonate shell.  In order to do so, they pull calcium and carbon from the water.  Two-part dosing is the process of adding calcium and alkalinity back into your aquarium, so that your invertebrates have the elements they need to build their skeletons/shells.

If you have a FOWLR system, you probably won’t have to worry about this, but if your tank is heavily stocked with corals and other inverts, then at some point you’ll have to two-part dose your system.

b) Trace Elements

Even though trace elements are found in less than 1ppm of seawater, some of them are essential for life.  If you don’t do regular water changes, then you will want to consider adding trace elements into your tank on a weekly basis.

c) Coral Color & Nutrition

There are several products on the market that purport to improve coral color and growth.  I can’t vouch for any of them, because I’m not a scientist.  But I do know many hobbyists that swear by these products.

d) Biolfiltration

Biofiltration additives can come in the form of vodka dosing, carbon dosing, and/or nitrifying bacteria.  I use Dr. Tim’s One and Only to help cycle my tank quicker.

But you could do regular carbon dosing to really boost your biological filtration.  By “feeding” the beneficial bacteria carbon, they are encouraged to multiply and can be sustained at higher levels.  This can potentially lead to a more robust biological filter, which in turn means your tank can carry a larger livestock load.

VI) Episode 05