Alex Bell transforms a super shallow aquairum into a naturalistic Rio Negro-inspired sandbank, designed specifically for bottom-dwelling species. This setup showcases the beauty and behaviour of Hoplisoma duplicareum in a minimalist blackwater environment.
Shallow aquariums are celebrated for panoramic, lush aquascapes that extend above the waterline. But for me, the appeal of a shallow tank lies in stripping back the greenery to recreate a more focused, subtle environment: a fallen-leaf riverbank
This project was the result of a 15-year dream to house one of my favourite catfish species. The duplicate cory (Hoplisoma duplicareum) was formerly in the genus Corydoras until a revision separated the genus into several genera. You will likely see them still called Corydoras duplicareus both online and in your local fish stores, though.
All photos by Alex Bell
The Concept: Why Go Shallow?
When designing an aquarium for a catfish that rarely leaves the substrate, a tall tank adds little value. In fact, vertical space can often distract the eye from the species you want to feature. Instead, I wanted a large expanse of sand for the duplicate corydoras to forage over, prioritising footprint over volume.
The WIO AquaVista 80 Super Shallow Pro, measuring 80×45×16cm (31x18x6”) – although current models are now 14cm (5.5") high – offered the perfect dimensions for this idea. The inspiration came from the meandering streams and small rivers of the upper Rio Negro basin. The aim wasn't a strict biotope aquarium, but a natural home inspired by these sorts of habitats.
Building the Sandbank
Creating a sloped sandbank in a shallow environment requires careful structural planning to prevent the "slope" from becoming a "flat" over time. I created foundations for the slope using WIO Bumblebee Stone (now called WIO Storm Stone). The colour of this stone would look natural if the sand shifted, exposing the rocks.
As for the substrate, it needed to be soft for sensitive barbels. I chose WIO Tigris Sand for its diverse grain sizes and warm yellow-orange hue, which would highlight the tint of blackwater. Furthermore, it is inert, meaning no buffering of the water chemistry, ideal for these soft water fish.
At the back of the sandbank, instead of sand, I used crushed Lava Rock and some Oase Scaper Soil. This created a small planting pocket for hairgrass to grow out of the water, mimicking the plants growing along the river's edge.
Botanicals and Blackwater
While the initial plan was for a rather barren sandbank, my vision evolved after watching footage of fish congregating around fallen wood in the wild. I chose to include this element into the scape. I utilised various aquarium safe woods to mimic fallen branches and exposed roots.
A good amount of botanicals (leaves, seed pods etc.) would complete the natural look. I chose the following species each serving a distinct purpose:
- WIO Cyca leaves – while not biotope correct, their spiky texture resembled decaying palm fronds on a minute scale.
- Live Oak Leaves – durable and small, they helped fill out the leaf bed while maintaining a sense of scale.
- Magnolia leaves – these were locally collected (free!) and their larger size would provide additional shelter for the catfish.
- Jacaranda and Kielmeyera seed pods – added a chunky texture and added a South American feel to the botanical bed.
- Betta Tea – As this is a blackwater aquarium, the tannin levels are regularly ‘topped up’ using a Betta Tea bag from Betta Botanicals.
Filtration and Adjustments
In this shallow aquarium, having equipment inside the tank would take away from the natural aesthetic. Plus, finding an internal filter powerful enough to get good circulation, while small enough to fit in the shallow water, would be extremely difficult.
The Oase FiltoSmart Thermo 100 proved to be the perfect choice. Taking both the filter and heater out of the display would keep both the aquarium's water and the overall look nice and clean.
The intake that comes supplied with the filter was too tall for the shallow tank, so a DIY modification was needed. I cut the intake pipe down to fit the limited vertical space. While it was successful, this was later replaced by some nano glassware for even better aesthetics.
Another adjustment came in the form of the marginal plants. The Eleocharis failed to thrive in this environment, so a species of fern available as a houseplant was added to the top of the sandbank instead. This added the pop of greenery above the waterline that I had envisaged.
The Stars of the Show: Hoplisoma duplicareum
The primary inhabitants are a group of nine duplicate corydoras. This species looks incredibly similar to Adolfo’s cory (Hoplisoma adolfoi). While you can tell them apart, it is not the simplest of tasks. The duplicate is generally stockier with a wider black band and – most accurately – they have serrations on the edge of the pectoral fin spine.
Duplicate corydoras are not the cheapest option from the cory family. This display would work just as well for many of the other common species of Corydoradinae. Read about our top five corydoras for beginners!
To add a "pop" of colour, a shoal of cardinal tetras (Paracheirodon axelrodi) were added. They naturally occur alongside these corys in the Rio Poranga, and their slim body shape suited the shallow water.
Top Tip: Transporting Corydoradinae
Corydoras catfish require extra care during transport:
Avoid Stress: They can "self-poison" when stressed, releasing toxins into the water. If you notice foaming at the surface of their bag, ask for them to be repackaged immediately.
Double Bag: Their sharp spines can easily puncture single bags.
Tape the Corners: Tape the bottom corners of the bag so the fish don't get stuck in the folds
Duplicate Corydoras Fact File
Scientific name: Hoplisoma duplicareum
Size: up to 5.5cm (2.2”) in captivity.
Origin: Rio Poranga, a small tributary of the upper Rio Negro, Brazil.
Habitat: The type locality had a soft sand substrate with flowing, tannin-stained water.
Tank size: 70x30x15cm (28x12x12”)
Water requirements: Soft and acidic with a pH below 7.5 and hardness below 10°H.
Temperature: 24-28°C (75-82°F)
Temperament: A shoaling species best kept in groups of at least six. Avoid housing with large fish that may try to eat them or territorial bottom dwellers.
Feeding: Unfussy omnivores, scouring the substrate for edible morsels. Offer a varied diet of prepared tablets and pellets along with meaty foods like bloodworms and Tubifex.
Availability & cost: Fairly common, expect to pay upwards of £16 each.
Final Thoughts on the Sandbank Biotope
This is my favourite botanical method aquarium that I have created thus far. Unique in its appearance, and its challenges, this aquarium has taught me plenty along the way. It has finally allowed me to keep a species I’ve coveted for years.
By housing them in a blackwater habitat, I see behaviours that I might not have otherwise been able to witness. They regularly search through and push botanicals out the way when searching for food, just like they would in the wild. They are clearly at home in this display, as they have just started breeding in here as well, so what was initially a biotope inspired project may now turn into a breeding project as well.
The ScaperSoil unfortunately acts in a similar way to activated carbon, removing the tannins from the water. This means I need to add more tannins frequently to maintain the tinted water. In time, I plan to replace the soil substrate with an inert sand given the hairgrass hasn't grown anyway.
Tai Strietman has also explored shallow water aquariums with Corydoradinae, opting for a smaller 45cm (18") setup with more emergent greenery, using peace lilies. His aquarium features one of the dwarf species, the pygmy cory (Gastrodermus pygmaeus), whose small size makes them ideal for nano environments.
This aquarium was first published in the February 2026 issue of Practical Fishkeeping. Subscribe today to never miss an issue!