RO/DI, Salt Mix, and Aquarium Heaters | Saltwater Aquarium Beginner's Guide: Episode 3
If you are following along with this Saltwater Aquarium Beginner's Guide, this video from Matthew is a big one. He breaks down the essentials of water, salt, and heat in a way that makes the early stages of reefing feel a lot less intimidating. Matthew explains why tap water can cause long-term issues, how an RO/DI filter works, and when store-bought RO or distilled water can be a good starting point. He also walks through choosing a salt mix, mixing and heating seawater the right way, and a few simple tricks to fill your tank without kicking up a sandstorm. The video wraps up with a clear explanation of heater sizing, placement, and using a temperature controller for long-term safety and stability. If you are just getting started in the saltwater hobby, Matthew’s overview gives you exactly what you need to move forward with confidence.
Using the Right Water for Your Saltwater Aquarium
One of the best pieces of aquarium advice I've ever been given is, "the water in your aquarium will never be better than the water you start with." This has stuck with me and I've put an increased level of importance on starting with the best water ever since. For this build, we're going to be making our own RO/DI water using a BRS RO/DI System.
But if you're just getting started, you might ask yourself... what types of water can I use for a saltwater fish tank?! With so many options, let's break down some of the most common options.
Different Types of Water
Tap Water - ❌ Not Recommended
As the name suggests, this is water straight out of your home's faucet. Tap water quality varies wildly across the country and can even be significantly different from one season to the next. We avoid using tap water in the saltwater aquarium hobby as it can have unwanted contaminants, chlorine, nutrients, and other elevated water parameters that can cause problems in our tanks. Even if it doesn't cause an issue from day one, there are contaminants that can build up in your aquarium over time. Those issues often go unaddressed until it's too late. Your aquarium's inhabitants may be alive, but nowhere near thriving.
Purified Water - ❓ Depends on Purification
Purified water is a tough one because whether or not it's safe for our use is dependent on the method used to purify the water. We generally don't recommend purified water unless you know how it was purified.
Distilled Water - ✅ Safe
Distilled water is created by boiling water into steam and letting it condense back into liquid water, leaving impurities behind. This process removes things like minerals and heavy metals that could cause problems in our aquariums
Reverse Osmosis (RO) Water - ✅ Safe
Reverse Osmosis water is water that has been filtered through multiple layers of a thin film that remove a majority (usually 96-99%) of contaminants such as salts, bacteria, heavy metals, and other organic impurities. This high rejection rate makes it a good option for filtering most tap water.
RO/DI Water - ✅ Best Option
Reverse Osmosis Deionization (RO/DI) water starts out as regular RO water, but is filtered through a final deionization stage that removes any lingering contaminants. This is the gold standard for water filtration in the saltwater aquarium hobby. When water leaves an RO/DI system, it has zero total dissolved solids (TDS).
Frequently Asked Questions About Water Filtration
Question: Which type of filtered water is best for a saltwater aquarium?
If you're looking for the BEST option, it's RO/DI water.
Because source water quality varies so much, RO/DI takes the guesswork out of it all. Using RO/DI water means starting with the best water possible. No questions in the back of your head when you run into a problem if the water you're mixing your salt with is the culprit.
Question: Where can I get RO/DI Water?
You’ve got a couple of easy options:
- Buy RO/DI water from a local fish store. Some fish stores may even sell RO/DI water that is pre-mixed with saltwater, making it a good option for beginners who don't want to invest in buying their own RO/DI system or have to worry about mixing their own saltwater.
- Make RO/DI water at home with an RO/DI filtration system. These systems are easy to install, relatively inexpensive, and make it super convenient to have RO/DI water on-hand without having to make a special trip to the fish store.
Question: How Does an RO/DI System Work?
An RO/DI system has 4 main filtration stages
- Sediment Filter: Removes larger particles such as dirt and sand.
- Carbon Block: Helps to remove chlorine, chloramine, odors, and volatile organic compounds (VOC)
- RO Membrane: Clean water is forced through the membrane layers, while rejected water is expelled out the waste line of the system.
- DI Resin: The final polish. Already-low TDS water leaves the RO membrane and passes through the DI resin, resulting in 0 TDS filtered water.
Learn More: How an RO/DI System Works
Question: Are RO/DI Systems Hard to Set up?
Actually, no!
RO/DI systems can be installed just about anywhere in your home like a bathroom, mudroom, or laundry room. Basically anywhere with a water hookup that doesn't get below freezing. Even if you don't have any prior plumbing knowledge, it's super simple.
Learn More: How to install a BRS 4-Stage RO/DI System
Picking the Right Salt Mix
For a saltwater aquarium, table salt isn't going to cut it. In addition to salt, natural saltwater contains a whole bunch of major, minor, and trace elements that are critical in keeping our fish, corals, and other invertebrates alive and thriving.
You might be asking, "which salt mix should I use for my first saltwater setup?" and the simple answer slightly unsatisfying. Any salt mix specifically designed for saltwater aquariums will work. From the classic Instant Ocean mix to more high-end options like Tropic Marin Pro Reef or Nyos PURE, they'll all be sufficient for your first reef tank. Of course, the more expensive salt mixes do have legit advantages, but when you're just getting started, I think it's best not to get caught up in the details. For this build, we'll be using Tropic Marin Pro Reef Salt Mix.
Before we go any further, let's talk about measuring the salinity level of saltwater. There are several different devices out there from handheld refractometers to more advanced and convenient digital testers. Most of us started out with the reliable, yet inexpensive BRS Handheld Refractometer. Simply drip a few drops of your saltwater onto the glass prism, close the lid, and look through the refractometer. If your eyesight isn't the best, the Hanna Marine Salinity Tester or Milwaukee Digital Refractometer are solid choices (and are what most of us long-term reefers usually upgrade to eventually anyway).
Drops of saltwater on a BRS Handheld Refractometer
Example salinity reading using a BRS Handheld RefractometerHow Do I Mix Saltwater for My Aquarium?
Mixing saltwater is one of the most basic but important parts of maintaining a healthy reef aquarium. We’ve mixed hundreds, probably thousands, of batches over the years and have learned a few things that make the process quick, consistent, and easy to repeat.
- Gather your supplies. You’ll need a clean Brute container or 5-gallon bucket(s), RO/DI water, salt mix, a small pump or powerhead, a small heater, measuring cup or kitchen scale, and a refractometer or digital salinity tester.
- Fill the container/bucket with RO/DI water. Leave a little room for the salt without overflowing.
- Add the pump and heater, then plug them in to start circulating and warming the water.
- Slowly add salt to the bucket as the water circulates. As a general rule, about 1/2 level cup per gallon will get you close to the target salinity without going over.
- Let the salt fully dissolve, then check the salinity. Your goal is around 1.025 specific gravity (~34 ppt) for a mixed reef.
- Adjust if needed. Add more salt if it’s too low, or a bit of RO/DI water if it’s too high. Give it time to mix fully between adjustments for accurate readings.
Tip: For consistent results, use an inexpensive kitchen scale to weigh your salt in grams instead of relying on measuring cups.
Important: NEVER mix salt in an aquarium containing animals.
Evaporation
One thing to know about saltwater is that as it evaporates, the salt and other minerals stay behind. This means that you should top off your aquarium with fresh RO/DI water (NOT saltwater). If this sounds like a hassle, stay tuned for Episode 4 where we'll talk about auto top off systems.
Using the Right Aquarium Heater
This part is pretty self-explanatory. Aquarium heaters do exactly what their name suggests. They heat the aquarium water to a specific set point, in our case, between 76-80°F. For this build, Matthew is going with the BRS Titanium Heating Element paired with the InkBird WiFi Heater Controller. Using a heater controller like this one has several advantages such as audible alarms and notifications to your phone if the temperature is out of range.
If you're new to the aquarium game, it might seem like there are a million different heater options, so let's break it down by what the heater is made out of and how the heater is actually controlled to know when the aquarium water needs to be heated:
Glass vs Plastic vs Titanium Aquarium Heaters
Glass: Most common in the aquarium hobby. Glass heaters are the least durable, but also the least expensive. The larger wattage sizes can be quite long, making them challenging to fit in certain spaces.
Plastic: Not as common as glass heaters, but they're a step up in durability. Most plastic heaters are on the smaller side, making them a good option for tight spaces, but are usually not powerful enough to heat large aquariums.
Titanium: Arguably the best heating option out of the three. They're very durable and transfer heat to the surrounding water quickly.
Pre-Set, Adjustable, and External Temperature Control
In addition to the material the heaters are made of, there are a few different options for how the heater works to heat the water.
Pre-Set: These heaters have built-in thermostats and are usually set to a specific temperature like 77°F.
Built-In Control: These heaters have some level of adjustability. Some can be set digitally while others have simple analog control.
External Controller Required: Heaters that have no control are simply on at full power when plugged in. This means that they'll need to be plugged into a dedicated heater controller. This could be as complex as a full Apex System or as simple as an InkBird Heater Controller.
Glass Aquarium Heater with Built-In Adjustability
Plastic Pre-Set Aquarium Heater
Titanium Aquarium Heater with No Built-In Thermostat
Coming Up Next
Episode 4: Cycling, Lighting & ATO
Adam has been reef keeping since 2010 and got his start on the Bulk Reef Supply Customer Service team. He believes the best tanks are built around what you enjoy, not what’s trending. Adam is especially interested in simplifying the hobby, exploring the latest gear, and helping others focus on long-term success over chasing perfection.
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