How to Set Up a 20 Gallon Saltwater Aquarium

Starting a new saltwater aquarium can feel intimidating, especially if it’s your first one. Here we’ll walk you through how we planned a 20 gallon mixed reef tank setup. It’s not a budget build, but it’s also not packed with high-end equipment. The goal is to land right in the middle, using solid, proven gear without adding extra complexity. Sometimes you just need something that works.

Step 1: Picking the Right Tank and Stand

The foundation of this build is a 20 Gallon JBJ Rimless All-In-One Aquarium paired with the matching JBJ Stand.

Why we picked this tank:

For this build, we wanted something that didn’t require drilling, extra plumbing, or a sump, but still gave us enough room to run proper gear. The JBJ 20 AIO hits that sweet spot. The standout feature is the rear chamber, which is a full six inches wide. That might not sound like much, but most all-in-one aquariums only give you about three inches to work with. The extra space gives us real flexibility when it comes to equipment, which is a common pain point with smaller AIO tanks.

The rear chamber is split into three sections:

• 1st Chamber: Mechanical filtration
• 2nd Chamber: Flexible chamber (perfect for a protein skimmer)
• 3rd Chamber: Return pump & auto top off sensors

The included return pump is more than capable for this size tank, and the first chamber comes with a large foam pad that provides excellent mechanical filtration right out of the box.

The stand itself is simple and functional. It fits the tank perfectly and offers just enough space underneath for an auto top off reservoir.

Step 2: Setting Up the Filtration

Filtration on this build is straightforward. The JBJ’s built-in rear chamber gives us a good starting point, with mechanical filtration handled in the first chamber. The included foam pad catches larger particles and detritus before the water moves through the rest of the system.

On top of that, we’re adding an AquaReady NF-1 Nano Protein Skimmer. A protein skimmer isn't required for success, but it will help remove dissolved organics, improve gas exchange, and allow us to be a little less reliant on water changes.

The compact footprint of the NF-1 fits the JBJ’s middle chamber perfectly. It's powered by a Sicce pump, meaning that it'll be reliable and quiet and the magnetic mounting system makes height adjustments easy. Overall, it gives us dependable performance without adding complexity, which fits exactly what we’re going for with this build.

Step 3: Installing a Heater

Keeping the temperature stable is key for any reef tank, and a reliable heater is one of the most important pieces of equipment you can buy. For this 20-gallon system, we’re looking for something compact, accurate, and built to last.

We’re using the Helio PTC Smart Heater for this build. It’s small enough to fit neatly in the rear chamber, and because it uses PTC (positive temperature coefficient) heating elements instead of a traditional resistance wire, it provides more consistent and safer heating over time. The titanium housing adds durability, and the waterproof connections give extra peace of mind in a small all-in-one system. As a bonus, it comes with a magnetic bracket, which makes mounting the heating element easy, clean, and most importantly, safe.

Tip: NEVER place a heater in the return pump chamber as the water level can fluctuate there and expose the heating element to air.

Step 4: Installing the Auto Top Off (ATO) System

What is an ATO and why does evaporation matter? When water evaporates, the salt stays behind. This causes the salinity levels to rise, which can quickly stress out your livestock. Manually topping off the tank can cause sudden swings in salinity, especially if you add too much freshwater at once. An auto top off system adds small amounts of freshwater automatically, keeping salinity stable and saving you from daily top-off maintenance. It also gives you some peace of mind when you’re away from the tank for a few days.

While an ATO is technically optional, having one is a non-negotiable around here. For this build, we’re using the Tunze Osmolator 3 Nano. The Osmolator and Osmolator Nano are two of the most trusted ATO systems in the hobby thanks to its reliable optical sensor, quiet DC pump, and overall durability. The included sensor shroud helps keep snails and other inverts from interfering, which is especially important in an all-in-one where tiny snails can more easily crawl into the rear filtration chambers. Just use the included magnet to mount the sensor in the return chamber at the desired water level height.

Tip: Place the ATO sensor in the return chamber as this is the only place where water level will change with evaporation. Usually you want the water level to be about an inch below the chamber before it so the water "falls" into the next chamber. This helps reduce stagnant areas and stops surface film from forming.

Auto Top Off Reservoir

Paired with the Osmolator Nano is the 5-Gallon Simplicity ATO Reservoir. A reservoir is simply a container that holds freshwater that will be used by the ATO system to replace evaporated water in the main display tank.

A few things we like about this reservoir:

• Front-fill design makes refills quick and easy without having to slide it out of the cabinet.
• Pre-cut openings for tubing and power cords keep things tidy.
• Solid acrylic construction holds up well over time.
• Sized perfectly for the stand and doesn’t eat up extra space.

Step 5: Mounting the Reef Lighting

Because this is going to be a mixed reef tank, we'll need some lighting that will support the health and growth of our corals. Additionally, lighting can also set the overall look of the tank in a room. For a 20-gallon mixed reef, we want a light that provides good coverage, proven performance for coral growth, and is easy to work with.

For this build, we’re using a Kessil A160WE Tuna Blue mounted on a Kessil Gooseneck Mount. The A160WE is a popular choice for nano tanks because it delivers excellent PAR (Photosynthetically Active Radiation), which is a measurement of light intensity, and also has a well-balanced spectrum that supports coral growth and color.

One of the signature features of Kessil lights is their shimmer effect. Shimmer refers to the rippling light patterns you see on the sand and rocks, similar to how sunlight looks underwater in the ocean. It gives the tank a natural, dynamic look that more diffused light sources have a hard time replicating.

The A160WE can be controlled manually using the built-in knobs, which is simple and reliable, or paired with a Kessil Spectral Controller X for more precise control over intensity and color. For this tank, we’re going to use the Spectral Controller and mount the light about 12 inches above the water surface.

Step 6: Aquascaping the Tank

If you ask me, this is where the real fun begins! But if you're new to aquariums, you might be asking yourself, what is aquascaping?

Aquascaping is just the process of arranging your rocks in a way that looks visually appealing, allows for future coral placement, promotes good water flow, and gives your future fish areas to establish territory and hide in. 

For this build, we’re using CaribSea LifeRock. It comes in interesting shapes that are easy to stack and glue together, which makes creating arches and shelves pretty simple. LifeRock is dry rock that’s coated in a bacterial film to help kickstart the biological filtration process once the tank is running. It also has a natural purple coloration that mimics coralline algae, which makes the tank look more established right from day one compared to plain white rock. The rock has plenty of ledges and pockets, which can make coral placement easier down the road too.

Tips for Aquascaping a Nano Aquarium

• Plan for coral growth: Leave enough open space so corals have room to grow without shading each other or growing into the glass.
• Keep flow in mind: Arrange the rock so water can circulate around and through the structure to avoid dead spots.
• Make it stable: Glue or epoxy pieces together so nothing shifts once you start adding livestock.
• Think about maintenance: Make sure you can still reach into the tank easily to clean the glass and siphon around the rock, including behind the rock structure.

Step 7: Selecting the Substrate

Substrate is just a fancy word for sand. It not only gives the tank a more natural look, but it also provides surface area for beneficial bacteria. For this build, we’re using Aqua Natural Coarse Aragonite. This dry aragonite substrate is collected from the Bahamas and has that bright tropical reef look that really makes the aquascape pop. The coarser grain size helps keep it from blowing in the flow, which is great for a mixed reef setup.

Step 8: Seeding the Tank with Beneficial Bacteria

Once the rock and substrate are in place, it’s time to seed the tank with beneficial bacteria. This is a crucial step in starting the nitrogen cycle, which is what makes the tank safe for fish and corals over time. Bottled bacteria products introduce the microorganisms that break down waste, convert harmful ammonia to less harmful nitrite and nitrate.

For this build, we're using PNS Deep Cycle. This is a unique trio of bacteria based on strains collected from actual reef environments. One of the nicest things about PNS Deep Cycle is that the label clearly lists the bacterial strains, so you know exactly what you’re adding to your system.

Adding bacteria at this stage helps jump-start the cycling process so the tank can handle livestock more efficiently once it’s ready.

Tip: For best results, soak your substrate with PNS Deep Cycle before adding it to the aquarium.

Step 9: Filling the Tank with Water

The moment you've been waiting for: it's finally time to fill the tank with water. But wait, not just any water! The quality of the water you use from day one plays a huge role in avoiding nuisance algae and keeping your reef healthy long term.

Why You Should Use RO/DI Water

For mixing saltwater, RO/DI (Reverse Osmosis/Deionization) water is the gold standard. An RO/DI system removes contaminants like chlorine, chloramine, silicates, nitrates, and phosphates that can cause headaches down the road. Investing in a basic RO/DI unit is one of the smartest long-term moves you can make in this hobby. It saves trips to the store, gives you full control over your water, and pays for itself over time.

Buying Water from a Local Fish Store

If you’re not ready to invest in an RO/DI system yet, that's ok! Most local fish stores (LFS) sell RO/DI water (and sometimes even premixed saltwater) by the gallon. This is a fine option to get started, just make sure to bring your own clean containers and test the water’s salinity and TDS (Total Dissolved Solids) occasionally. Only buy water from a store you trust to properly maintain their RO/DI filtration system.

Mixing Saltwater at Home

If you’re mixing your own saltwater:

1. Gather your supplies. You’ll need a clean Brute container or 5-gallon bucket(s), RO/DI water, a reef salt mix, a small pump or powerhead, a small 50 watt heater, measuring cup or kitchen scale, and a refractometer or digital salinity tester.
2. Fill the container/bucket with RO/DI water. Leave a little room for the salt without overflowing.
3. Add the pump and heater, then plug them in to start circulating and warming the water.
4. Slowly add salt to the bucket as the water circulates. Follow your salt mix manufacturer’s instructions, but as a general rule, about ½ level cup per gallon will get you close to the target salinity without going over.
5. Mix and check. Let the salt fully dissolve, then check the salinity. Your goal is around 1.025 specific gravity (~34 ppt) for a mixed reef.
6. 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. It removes the guesswork and helps you hit the right salinity every time.

Filling the Tank

Once your saltwater is ready, you can start filling the tank. Pour the water slowly over a plate or plastic bag placed on the substrate to avoid stirring up sand and clouding the water (some cloudiness is inevitable).

Step 10: Let There Be Flow!

Once the tank is filled, it’s time to bring everything to life. Start by turning on the return pump, keeping an eye on the water level in the rear filtration chambers as the system stabilizes. Add more saltwater if the water level drops below your ATO sensor right away (all future top-offs due to evaporation should be done with freshwater).

Once the water level is correct in the rear filtration chambers, plug in your heater to maintain your target temperature (usually around 78°F for a mixed reef). Let the system circulate for a few hours to clear up any remaining cloudiness from the sand and rock.

Final Tips

• Check every connection one more time, including hoses, power cords, and mounts, to make sure everything is secure.
• Set your ATO to the proper water level in the return chamber and make sure it activates correctly.
• Turn on your skimmer last or leave it off for a day or two to give the beneficial bacteria a head start.
• Wait to plug in your lighting until the tank is more established. This helps reduce the chance for nuisance algae.
• Monitor temperature and salinity over the first 24 hours to make sure everything stays stable.

A Full List of the Gear on This Build

Ready to add your first fish or coral?

Let's make sure you're armed with all of the right information to make your first fish addition a successful one!

Learn More: How To Acclimate Live Fish & Corals Into A Saltwater Aquarium