Phil's final snorkel in the Amazon revealed an extraordinary clear-water habitat, carpeted with Eleocharis and packed with shoals of tetras, cichlids and Hemiodus. Could this be one of the ultimate South American biotopes?
Our guides suggested we head downstream towards a lagoon for our final snorkel of the expedition. It proved to be the perfect finale – and arguably the most spectacular fish-watching experience of the entire trip.
It was the last day of our DC Expedicoes trip to the Amazon, and before this trip, I felt I had a good understanding of what an "Amazon Biotope" should look like. Our early snorkels delivered exactly that. Classic blackwater, with cola‑coloured water, soft substrate made of layered leaves, and twisted branches lying like dark sculptures on the riverbed.
A group of pencilfish in a blackwater habitat we previously visited - Photo by Gustavo Fonseca
We had, however, already encountered a completely unexpected, clearer, rocky, biotope earlier in the week – an amazing underwater world with sandy shallows, bright visibility and limited plant life. You may have read about this habitat in the March 2026 issue of Practical Fishkeeping.
Yet, even this unique and unexpected habitat could not have prepared us for what came next.
An Amazonian Blue Lagoon
As the current carried us downstream, Fernando Henrique, a good friend from the expedition, and I led the way while the rest of the group continued exploring an area of rapids we had previously been exploring. After days of dark, tannin-stained Amazonian waters, we had little idea what was waiting around the next bend.
Then the river opened into a warm, crystal-clear lagoon.
The transformation was extraordinary.
Beneath the surface was a vast turquoise pool, its pale sandy bottom covered in vivid green hairgrass (Eleocharis sp.). It looked less like a remote Amazonian waterway and more like an enormous, perfectly maintained aquascape – except that every plant, fish and fallen leaf was entirely natural.
A shoal of slender Hemiodus in the blue lagoon - Photo by Phil Towers
As we approached the lagoon entrance, our first inhabitants appeared. A shimmering wall of slender Hemiodus, including at least 50 full-sized adults, glided calmly out of the pool and towards the main river.
Inside, the scene became even more impressive.
Large shoals of fish circled the almost perfectly circular central basin, weaving above the underwater meadows. After six hours of almost continuous snorkelling, the warm 27–28°C (81–82°F) water was also a welcome relief.
Related article: An introduction to 3 awesome and rare tetras.
Hairgrass meadow - Photo by Phil Towers
Underwater grasslands
The hairgrass meadows were the heart of the lagoon, providing shelter for an astonishing variety of fishes.
Large shoals of Curimatopsis pallida hovered among the bright green blades, darting into the vegetation whenever we disturbed them. Vast groups of red base tetras (Hemigrammus stictus) were also present, their red caudal peduncles glowing against the green vegetation.
Among them were similar-looking but more elongated characins, lacking the shoulder spot and displaying an even deeper red blaze. These were later identified as Brittanichthys axelrodi.
Brittanichthys axelrodi - Photo by Gustavo Fonseca
The behaviour of the red base tetras was particularly interesting. Earlier in the expedition they had generally appeared as fleeting, individual fish, but here they seemed remarkably confident, swimming openly in large groups.
The lagoon's grassbeds were home to a remarkable collection of South American fishes, including Apistogramma elizabethae, Apistogramma gephyra, dwarf and barred pike cichlids, flag cichlids, checkerboard cichlids, spotted headstanders and severums.
Threadfin acara (Acarichthys heckelii) were another highlight. The males were especially impressive, with spectacular black-and-pink extensions at the front of the dorsal fin, giving them an unmistakable punk-rock appearance.
Meanwhile, striped barracuda tetras, dwarf freshwater barracuda and pike characins patrolled the open water above the grass, adding a predatory element to this otherwise tranquil-looking habitat. These would be best left out of your biotope aquarium if you plan to keep smaller fishes.
Threadfin acara - Photo by Gustavo Fonseca
A natural giant aquascape
For fishkeepers, the lagoon was fascinating for another reason. Its appearance demonstrated just how closely a carefully designed aquarium can sometimes resemble a natural habitat.
The fine, pale river sand provided a striking contrast to the intense green of the Eleocharis. Fallen leaves were scattered across the substrate, breaking up the expanses of grass and providing organic material and nutrients.
Rather than forming a uniform carpet, the vegetation created a patchwork of open sand, dense grass and leaf litter. Fish used each of these areas differently, moving between them as they fed, sheltered and interacted.
The result was a highly structured habitat – one that offered plenty of inspiration for anyone attempting to recreate a South American biotope aquarium.
The Blue Lagoon - Photo by Phil Towers
Related article: Life in the Super Shallows: Recreating a Rio Negro Sandbank.
Heart-stopping encounters
The lagoon wasn't occupied exclusively by fish.
Twice, while swimming above the grassbeds, I became aware of sleek shapes moving across the bottom. Snakes.
Earlier in the expedition I had encountered Amazon coral snakes (Micrurus spixii), a highly venomous species that were certainly not animals I wanted to meet while snorkelling. The lagoon's snakes, however, were false coral snakes (Anilius scytale), which are non-venomous despite their convincing appearance.
False coral snake - Photo by Fernando Gustavo
One disappeared beneath the bark of a submerged tree, while another vanished into a mound of leaf litter. Both encounters provided a useful reminder that the lagoon was a living ecosystem rather than an enormous aquarium – and that there was plenty going on beyond the fish.
Nearby, a group of Lilith's eartheaters circled partially submerged trees around a small island. They repeatedly moved back and forth through the branches, seemingly curious about us but unwilling to remain still for long.
Fernando and I spent several minutes filming them as they moved through the shifting light, their bodies passing between gently swaying branches.
Related article: Which eartheater cichlid is best for me?
Satellite pools
The main lagoon wasn't the only habitat worth exploring. Small satellite pools branched away from the central body of water, separated by mud banks and arches formed from intertwined tree roots. Some of these secluded pools were just large enough to squeeze through, opening into pockets of exceptionally clear water.
One-lined pencilfish in sheltered pocket of the lagoon - Photo by Gustavo Fonesca
The temperature also seemed higher in these sheltered areas, although I did not measure it.
These smaller pools provided refuge for numerous small tetras. Larger fish were less abundant, although striped barracuda (Acestrorhynchus nasutus) moved confidently through the shallows, presumably searching for prey.
The mud banks themselves were reinforced by impressive pyramids of tree roots. These created a complex network of gaps and shelters, adding yet more structure to an already intricate habitat.
For small fishes, such areas are likely to provide valuable cover from larger predators while also creating calmer water away from the main lagoon.
Life in the lagoon
The Blue Lagoon supported an impressive variety of fishes. Some would clearly be unsuitable for a typical community aquarium, particularly the predatory Acestrorhynchus, but many of the smaller species could provide inspiration for a dramatic South American biotope.
Below is the full list of species we encountered in the lagoon:
- Threadfin acara (Acarichthys heckelii)
- Dwarf freshwater barracuda (Acestrorhynchus minimus)
- Striped barracuda tetras (Acestrorhynchus nasutus)
- Pike characin (Acestrorhynchus microlepis)
- Apistogramma elizabethae
- Apistogramma gephyra
- Blood-red tetra (Brittanichthys axelrodi)
- Spotted headstander (Chilodus punctatus)
- Curimatopsis pallida
- Checkerboard cichlid (Dicrossus filamentosus)
- Red base tetra (Hemigrammus stictus)
- Slender Hemiodus (Hemiodus gracilis)
- Barred pike cichlid (Lugubria lenticulata)
- Rosy tetra (Megalamphodus rosaceus)
- Flag cichlid (Mesonauta sp.)
- One-lined pencilfish (Nannostomus unifasciatus)
- Dwarf pike cichlid (Wallaciia notophthalmus)
Flag cichlids lurking in the Blue Lagoon - Photo by Phil Towers
A fitting finale
Time passed remarkably quickly in the Blue Lagoon. Eventually, it was time to leave the water, return to the boats and begin the journey back to our floating accommodation.
After days spent exploring Amazonian rivers and streams, the lagoon had provided an unforgettable finale.
Its clear blue water was a world away from the dark, tea-stained habitats I had expected to encounter in the Amazon. The combination of pale sand, lush Eleocharis meadows, submerged roots, leaf litter and huge shoals of fish created one of the most extraordinary freshwater habitats I had ever experienced.
For a fishkeeper, it was also a powerful reminder that some of the most spectacular aquariums on Earth don't need to be designed by an aquascaper.
Sometimes, nature gets there first.
The Blue Lagoon was one of three Rio Jauaperi biotopes Phil Towers outlined in his article in the July issue of Practical Fishkeeping Magazine. Subscribe today to never miss an issue.