How Do You Grow Cactus in an Aquarium?

Growing cactus in an aquarium is one of the best indoor gardening projects you can undertake. Cactus can thrive in the aquarium because it needs less water and adequate moisture. Besides, cacti prefer cool and dry surroundings and an empty fish tank can provide that environment. You will also monitor the cacti’s lighting, temperature, and water needs.

To grow cactus in an aquarium, clean an empty fish tank then arrange a 2 or 3-inch layer of clean pebbles or stones at the bottom. Add 4-5 inches of coarse sand or cacti soil on top of the pebbles. Plant your preferred cacti seed or propagate a stem from the parent cactus.

Cactus has short fibrous roots that do not spread widely or penetrate deeply into the soil. Pebbles and coarse sand will give its fiber-like root system adequate drainage and nutrients. The cacti growth rate is also pretty slow. Therefore, the minimum aquarium size to grow cactus is five gallons.

Use cactus in an aquarium to decorate your south facing windows.

Can a cactus grow in an aquarium (underwater)?

Cactus cannot grow under the water in an aquarium it is very succulent. The cacti stems will attract bacteria and fungi and rot quite fast if submerged underwater. But you can use an old fish tank as a greenhouse to plant a cactus terrarium.

There should be no water in the tank but only the prepared substrate and the cacti. Afterward, you will have to place the cactus terrarium in the correct temperature, sunlight, and humidity. This article will guide you on how to grow cactus in an aquarium.

How to plant and care for cactus underwater

Cacti thrive in arid and semi-arid areas. If submerged in water, the succulent cacti pads would attract fungi and rot or develop splits. However, growing and caring for a cactus in an aquarium is simple. Ensure the aquarium humidity for your cactus is between 40% and 60%.

Here are some conditions and requirements for cacti grown in an aquarium:

Soil/substrate

Soil and or substrates are the primary cactus growth requirements. The best soil for cactus is sandy soil, which is not only porous but also contains nitrates and inorganic matter. Add two layers of substrate for aquarium cacti: 4-5 inches of coarse sand and two or three inch layer of clean pebbles or stones.

Sandy soil also provides proper drainage and excellent aeration to cactus roots. Ensure that the sand soil is fast drying to prevent the roots from rotting and avoid oversaturation of the cactus cells. Sand and pebbles will provide structural support and hold moisture for the cactus inside the aquarium.

Light

Aquarium cactus require bright indirect sunlight. Avoid direct sunlight because it raises the aquarium temperature and might bake the cactus.

Place the aquarium close to the south facing window for warmth and adequate lighting. If you can’t find a good place with natural lighting, you can use artificial fluorescent light. For healthy aquarium cactus, leave the artificial lights on for 16 hours a day.

Temperature

Cacti varieties of the Opuntia family such as Opuntia compressa are cold hardy and can withstand freezing weather as low as -35°F. However, an aquarium cactus requires a temperature of 45°F to 85°F to thrive.

Too hot temperatures can bake the stems while freezing temperatures can cause rotting and death of most cactus cells. Generally, cacti will survive in an aquarium at your average room temperatures.

Fertilizer

To grow cactus in an aquarium, use a houseplant fertilizer that’s high in potassium and phosphorus. Avoid fertilizers that have a high nitrate ratio. Dissolve 1/16 teaspoon of a 10-15-10 fertilizer in one liter of water. Drip the liquid fertilizer in your cacti’s roots and return some soil to the roots.

Cactus does not have high nutritional needs. Usually, the pebbles and coarse sand offer enough nutrients necessary for the aquarium cactus. But if you decide to fertilize it, do so two or three times a year especially in spring, winter and summer.

Pruning

You can prune your cactus due to the following reasons;

Propagation: If you want the cactus to grow into a new plant. You can cut the huge limbs of the plant and propagate them into a new plant.
When the cactus has grown too tall: Prune it to your desired shape and size when your plant has grown beyond its required size.
When the cactus is damaged: Prune rotting or discolored cactus parts such as damaged leaves to contain the damage and stop it from spreading to healthy parts.

When pruning cactus, use protective leather hand gloves to prevent cactus spines. Use sharp sterilized tongs or scissors to prune cactus. Sterilized tools protect your cactus from bacterial and fungal infections.

Note that pruning of an indoor cactus is not common because they grow slow. That’s why, cactus in an aquarium requires low maintenance.

Watering

Pay attention to how much water your cactus needs. Just because it thrives in desert places does mean they require little water. Underwatering cactus in an aquarium may cause it to wilt. Similarly, Overwatering may cause stunted growth and death to cactus.

Water your cactus at least once a week in summer and spring. During autumn and winter, water the cactus when the substrate is dry. Ensure the soil is well soaked and allows easy drainage.

Where possible, use rainwater to water cacti. Tap water has accumulated minerals that may disrupt food and water intake from the soil.

What type of cactus can you grow in a fish tank?

The type of cactus to grow in a fish tank depends on the size of the tank among other things. The best aquarium cacti are those that remain small even when fully developed.

Here are some types of cactus to grow in a fish tank:

Bunny Ears Cactus (Opuntia microdasys)
Chin Cactus (Gymnocalycium)

Saguaro Cactus (Carnegiea gigantea)
Old Lady Cactus (Mammillaria hahniana)
Star Cactus (Astrophytum asteria)

Easter Cactus (Hatiora gaertneri)
Christmas Cactus (Schlumbergera bridgesii)

The above cactus are suitable for indoor decorations and projects. They grow up to a maximum of 8 inches, and are ideal for most tank sizes. They also produce beautiful flowers. For example, Easter and Christmas cactus blossom into attractive pink flowers.

Precautions and risks

Have you ever touched the cactus with your bare hands? You can nurse sore and scratchy painful wounds. Cactus have spikes and needles all over the plant. The spikes vary from cactus to cactus. Some spikes are visible; others are tiny and difficult to see or remove.

Cactus is also a delicate plant that can break easily if mishandled. The cactus pads can break and fall from the parent plant during transportation or planting. Therefore, take caution when dealing with cactus.

Here are risks and precautions when handling cactus.

Use gloves when touching the cactus.

Handling cactus with bare hands is risky. It will leave your hand bloody with spikes. The first prexaution when growing cactus in an aquarium is to use gloves made of persistent materials that cactus spikes cannot penetrate.

Leather and nitrile-coated gloves are the best. They are puncture resistant and more durable than other gardening gloves. Even with quality gloves, do not grab the cactus tightly. Some spikes are long and tough and might penetrate through the gloves.

Use tongs when cutting or pruning your cactus

Metal or plastic tongs are good when handling small cacti. You need to control the tongs properly to avoid damaging the cactus. Use tongs that have silicon to handle cactus in an aquarium. The tongs provide cushion-like support to the cactus and prevents significant damage when cutting or pruning cactus.

Hold the cactus firmly and not tightly when pruning or cutting it to avoid breaking it. You need gloves while handling the tong and the cactus.

Use rugs and carpets when moving

Matured cacti depending on the species, can be very heavy. Some cacti are also strong, with more robust spines, almost like thrones. You must be careful while moving these around the house or growing them in an aquarium. Otherwise, you may damage the plant or injure yourself.

Professional landscapers recommend a carpet or a rag for moving cactus around the house because it provides enough padding for the plant. Carpets also handle the heavyweight of the plant and prevent the plant from any damage. Other tools to use when handling cactus include chopsticks, towels, and newspapers.

Ensure you have the proper tools and protective gloves to manage it.

References

Perry, L. (2022). Growing cacti indoors. University of Vermont Department of Plant and Soil Science.
Texas A&M University. (n.d) Cactus. Department of Horticulture.