Straight answer — No, terrariums should not be used as aquariums.
Although the two might look similar, more often than not, they are structurally different. As a result, using a terrarium in place of an aquarium could result in leaks, poisoning, even the complete destruction of your tank.
Ideally, you should use a terrarium only for terrestrial pets and aquariums for aquatic pets.
But we don’t live in an ideal world, do we? Sometimes, an old reptile tank might be the best option available.
If you find yourself in such a situation, don’t worry, there’s good news:
With some improvements, some reptile tanks can also serve as fish tanks. Of course, provided your reptile tank meets some basic requirements that will allow you to turn it into a functional aquarium.
And how do you go about doing this?
Well, here’s an in-depth guide to converting your terrarium into an aquarium.
But first…
The Differences Between a Terrarium and Aquarium
Though similar in appearance, terrarium and aquarium tanks have one fundamental difference—they are designed to host animals from opposing habitats.
The word terrarium stems from the word “terra,” which means land. From this, we can deduce that terrariums host terrestrial animals.
On the other hand, aquariums start with “aqua,” which means water. Ergo, aquariums host aquatic animals.
Consider this brief comparison of the two tanks and the animals and habitats they typically support:
| Aquarium | Terrarium | |
| Habitat | Aquatic | Mostly Arid |
| Water | Yes | Little to None |
| Land | No | Yes |
| Plants | Yes (Aquatic) | Yes (Terrestrial) |
| Aquatic Animals | Yes | No |
| Semi Aquatic Animals | No | No |
| Terrestrial Animals | No | Yes |
Aquariums are designed with thicker glass and watertight seals. On the flip side, terrariums have thin glass and often feature a lid, ventilation holes, and a drain.
When picking a terrarium for conversion, go for one with the thickest glass you can find and no holes.
Thick glass offers structural integrity to your tank. This is critical to supporting the weight of the water once filled.
Another thing to watch out for is the silicone sealant on your tank.
Sometimes, the silicone sealant on some terrariums isn’t watertight. To confirm if the one inside your reptile tank is fit for the job, pour and hold a few inches of water in the tank and check for leaks.
Note: If your terrarium has holes or leaks, DO NOT use silicone from your local hardware store to seal it.
The type of silicone you typically find there contains toxic chemicals. When used on your tank, these toxins will leach into the water, putting your fish at risk.
If need be, look for sealants with 100 percent silicone. These should contain pure silicone without any additives.
Should because although some brands advertise 100 percent silicone, their fine print shows additives like fungicides.
If you notice any of the following in the ingredients list, DO NOT use the silicone on your aquarium:
- Mold-Free
- Anti-Microbial
- Bacteria Protection
- Mildew-Resistant
These will kill the beneficial nitrogen cycle supporting bacteria and, eventually, your fish too!
Related: How to Reseal a Fish Tank - Explained Step-by-Step
How to Turn Your Reptile Tank Into an Aquarium
Creating an aquarium from a terrarium isn’t easy. More so, if the terrarium you’ve got has:
- Breathing holes, a drain, or gaps in the sealant;
- A door, or other openings in the tank, or;
- Thin glass walls.
These would make the conversion a lot harder, if not downright impossible.
But if you’re lucky and have found a reptile tank without these caveats, then you’re free to go ahead.
However, this then begs the question—how do you turn a terrarium into an aquarium?
Well, here’s a blow by blow guide to start you off:
Step #1: Clean the Terrarium
Thoroughly clean the tank. This is especially important if the terrarium was previously home to a reptile.
Reptile waste is rich in ammonia, which is toxic to fish.
Remove all the substrate, rocks, gravel, ornaments, etc., from the tank.
These also harbor ammonia and other compounds that could upset the chemical balance of your aquarium. They could also introduce harmful bacteria and fungi to your aquarium.
Pro Tip: When cleaning, you should probably use non-toxic, biodegradable products. These are less likely to introduce more chemicals to the tank.
Additionally, be sure to scrub the seals and corners thoroughly—this is usually where harmful compounds hide.
Step #2: Verify the Structural Integrity of the Tank
Once clean, place your fish tank on a stand graded to support a significant amount of weight.
Water weighs about nine pounds per gallon. And you’ll be using a lot of it to fill up your tank. Your safest bet would be to use a specialized aquarium stand.
Additionally, depending on the manufacturer, you can also verify your tank’s ability to hold water. To do this, check the bottom of the tank for a manufacturer’s tag. If you cannot find one, you could contact them.
This information will inform the strength of support to use and the tank’s structural integrity.
However, without this information, a good rule of thumb is to not put water in a tank made solely for dry habitat reptiles.
These usually use thinner glass on the sides. Glass that would not be able to withstand the weight of the water.
You can also use this online tool to calculate your tank’s glass thickness and safety factor.
Step #3: Set up Your Fish Tank
Add in the substrate, gravel, decorative rocks, ornaments, even set up some live plants.
The top recommended planted aquarium greenery include:
- Java Moss
- Water Wisteria
- Dwarf Sagittaria
- Marsilea Munita
Fill your aquarium with water and add the appropriate amount of de-chlorinator to detoxify it. You can also use spring or deionized water if your tap water quality is wanting.
Set up the appropriate filter and hood according to the dimensions of your tank, then carefully plug them into a power outlet.
Depending on where you are, you might also need a heater when keeping tropical fish.
Related: Top 10 Low-Maintenance Aquarium Plants
Step #4: Initiate the Nitrogen Cycle
The nitrogen cycle refers to a biochemical process that takes place in the water to ensure the continuous breakdown of toxic fish waste and old uneaten food into harmless nitrate.
To ensure that the cycle is well established in your tank, you will need to test the water daily using ammonia test strips.
When you can no longer detect ammonia in the water, you will know that the nitrogen cycle is going well. You should then start testing for nitrites. When you can no longer detect nitrites, the cycle is complete, and your fish tank is finally fish-ready.
Related: How to Start Nitrogen Cycle in an Aquarium
Step #5: Introduce Your Fish to Their New Home
Upon successful establishment of the nitrogen cycle, you can place your fish inside their new tank.
Introduce the fish a few at a time, with at least a week’s break in between to avoid triggering an Ammonia spike.
Too big a fish community too soon will generate too much waste. More than the nitrogen cycle can handle, leading to dead fish.
Additionally, you should be careful not to overstock your aquarium. A good rule of thumb to calculate safe stocking levels is the one-inch per gallon rule.
That is one inch of fish for every gallon of water.
However, the fish inches rule merely offers a blanket solution that might not suit your specific conditions. To be on the safe side, you should probably adjust to one fish inch for every two gallons of water.
Learn More About Aquariums
Fishkeeping is one of the most fulfilling hobbies there is. Pet fish evoke a sense of calm and comfort.
The hobby also comes with a need for consistent and accurate information to ensure happy, healthy fish.
For more information on aquarium building, maintenance, and the fishkeeping hobby, visit Aquarium Sphere—your one-stop resource for all aquarium and fishkeeping advice.
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