How to Start a 21-Gallon Nano Reef Tank

Starting a nano reef tank can be an exciting and rewarding experience! We started with a TideLine 21 Gallon Aquarium with matching stand. Here’s a detailed guide to help you with the steps for starting a nano reef tank. It took us hours of research to find information about how we were going to start our nano aquarium. We wanted to make a website that put all of that research into one place and provide easy instructions for the typical beginner who may not have as much time to research as we have. 

Before you even set up the tank, it’s important to plan what you want to achieve. Nano tanks are small and require careful balance, but they can thrive with proper attention to detail. Here are some key points to consider:

•  Tank Size: A 21-gallon tank is a great choice for a nano reef. While nano tanks are typically under 30 gallons, a 21-gallon tank provides a good amount of space for fish, corals, and live rock.

•  Theme: Do you want a coral-heavy tank with just a few fish, or are you planning a more fish-centric setup with a few corals? Deciding on a theme will help you plan your livestock and equipment.

• Tank:  21 Gallon Tank  by TIDELINE also purchased the matching stand in black.

• Filtration System: A reliable filter is essential. You can go with a hang-on-back (HOB) filter or a canister filter. The filter will help with biological filtration and water flow. We used the pump that came with our 21 Gallon TIDELINE Tank from our local fish store.  We learned that it is not necessary to have a protein skimmer on this tank. You can if you want but it is not mandatory. 

• Heater: Marine life thrives in temperatures between 76°F and 82°F. A heater with a reliable thermostat is important. We use SICCE Contactless Heater (150W) 

• Lighting: LED lighting is highly recommended for reef tanks because it's energy-efficient and provides the correct light spectrum for coral growth. Aim for a light that offers both white and blue lights. We use the AI PRIME 16 Light. We have the black version but they also have white. You will also need the PRIME Arm Attachment. This light can be controlled from your Phone and was very easy to set up.  See our current light settings for the Prime 16.  

• Powerheads: These provide the necessary water movement to mimic ocean currents. Aim for 20-30 times turnover per hour in your tank. The pump that came with this tank would be sufficient enough to start out. We did buy a small 3 watt wave maker from Amazon just to have a little more movement due to my rock placement. 

• Live Rock and Live Sand: Live rock is crucial for biological filtration and creates a beautiful structure for corals and fish to interact with. Live sand helps with filtration and provides a home for beneficial bacteria. We used Carib Sea Arag-Alive Special Grade Sand, 20 lbs. You can choose how deep you would like the sand to be. The general rule of thumb is 1 inch sand bed = 1 pound of sand per gallon. For a 21-gallon tank: 20–25 lbs of live sand works well. 

• Create Height and Depth: Stack the live rock to create caves, overhangs, and open spaces for your fish to swim through. Ensure the structure is stable so rocks don’t collapse. Some aquariasts like to glue the rocks with a gel superglue. We used this GEL Gorilla Glue. You can technically use any GEL superglue, We’ve heard of people using Dollar Store brand gekl glue and it worked for them in their reef tank.. 

• Leave Swimming Space: Make sure there’s enough open swimming space for your fish and invertebrates. Creating holes and pockets is ideal. I will note that we let our aquascape sit for a week and changed the position of the rocks 3 times until we were satisfied with the set up. Don’t glue right away until you are happy with your lay out.

• Consider Coral Placement: Plan where your corals will go—ensure each type of coral has enough space to grow without crowding other species. Make sure to do your research and find out how much lighting each coral needs. 

•  Fill the Tank: Fill your tank with high-quality, saltwater. You can use pre-mixed saltwater from your local fish store or mix your own with marine salt and RO/DI water. We chose to use premixed saltwater from our local fish store. We bought these 5 gallon jugs to have filled up monthly. They have worked great so far. We purchased 3 of these from Amazon. We use 1- 5 gallon jug for RO/DI water from our fish store and 2-5 gallon jugs of premixed saltwater from the fish store. IMPORTANT: When your tank’s water evaporates (daily) you will need to top it off with RO/DI water because your saltwater becomes more salty as it evaporates. You want to make sure that your salinity is 1.025. We use this AUTOUTLET Salinity Refractometer to measure. 

• Cycling the Tank: This is the most important part of starting your reef tank. You need to establish beneficial bacteria that will break down harmful ammonia and nitrites. The cycling process usually takes 4-6 weeks.  We used Dr. Tim’s Aquatics Saltwater One & Only Nitrifying Bacteria  and then added Dr. Tim’s Aquatics Ammonium Chloride Solution for Fishless Cycling to start the cycle.   This reduces the amount of time before you can add fish and corals safely.  We like to do a fishless tank cycle. Some people prefer to use fish to start the cycling but you risk killing the fish. We didn't want the heartache or extra stress. 

• Testing Parameters: Regularly test for ammonia, nitrites, and nitrates. You should see ammonia spike first, followed by nitrites, and finally nitrates. We use the API Saltwater Testing Kit and the API Reef Testing kit. 

• Patience: During cycling, it’s important to wait until both ammonia and nitrites drop to zero, and you start seeing nitrates appear. Our  Nitrates were showing 20 PPM after cycling so we did a 15% water change and it came down to 10 ppm. 

Once the tank has cycled we added AlgaeBarn LIVE Copepods and Phytoplankton  you can find these on Amazon as well. I checked the pricing between ordering off of AlgaeBarn’s website directly or through Amazon and it’s actually cheaper to buy the set with both of the items on Amazon.  A HUGE shoutout to AlgaeBarn because I ordered the copepods and phytoplankton before my tank was cycled and I was worried that they would die sitting at room temperature but they lasted for 7 days with little die off.  You have to make sure you take the top off of the jar so they get oxygen and feed a couple drops of phytoplankton per day to keep them alive. 

Don’t put these Copepods in the fridge. However, you will have to store the Phytoplankton in the fridge. 

Clean Up Crew:

• Snails: We added 2 Snails like trochus or nassarius are great for cleaning the substrate.
• Crabs: We added 5 Hermit crabs because are excellent scavengers and will help with algae control. NOTE: We didn’t have enough algae to feed the crabs completely yet so we purchased some Hikari algae tabs and added one into the tank to give them more food until the Algae begins to feed them more. 

Corals: 

• When we purchased our clean up crew we also purchased 3 corals at the same time to start with. They include 1 Duncan Coral, 1 Mushroom Coral and 1 Zoa colony. 

• Salinity: Maintain salinity between 1.022 and 1.025 specific gravity.
• Temperature: Keep your tank between 76°F-82°F. We use this 2-in-1 Aquarium Magnetic Cleaner with Digital Aquariums Thermometer and digital temperature gauge from amazon. 
• pH: Keep the pH between 7.8 and 8.4. I will note that the PH test included in the API Saltwater Testing Kit is very hard to determine the exact color. This is the only aggravating test in that kit. 
• Ammonia/Nitrite/Nitrate: Aim for ammonia and nitrites to be zero, and nitrates under 20 ppm. We put a Seachem Ammonia Alert reader on our tank. This just helped our peace of mind with a quick glance during the beginning stages. 

• Regular water changes (about 10-20% per week) help maintain water quality and remove excess nutrients.

• Be mindful of algae growth, especially in the early stages. Regularly clean the glass and remove any excess algae.
• Use your clean-up crew (snails, crabs) to help with algae management.

• Fish: Feed your fish small amounts 2-3 times per day. It's better to feed them little and often.
• Corals: Many corals feed on phytoplankton and zooplankton. We use  Red Sea Reef Energy or you can use other coral-specific supplements. Be sure not to overfeed, as excess food can lead to poor water quality. We also have continued to dose live Phytoplankton from AlgaeBarn daily for the corals. We normally dose 20 ML per day into the tank. This has seemed to work well. 

Once everything is set up and stable, you’ll be able to sit back and enjoy the beauty of your nano reef tank! The vibrant colors of the corals and the playful behavior of your fish will provide endless enjoyment.

Starting a nano reef tank requires patience, but the rewards are worth it. With careful planning and maintenance, your 21-gallon tank can thrive as a beautiful, low-maintenance reef ecosystem. Just remember to test your water frequently, be mindful of your livestock, and enjoy the process of watching your tank grow and develop.