The Making of a Blackwater Tank


After having unsuccessfully attempted to extensively breed Sailfin tetras for over a year, I got annoyed, and felt that it was time to get serious. Inspired by setups used to breed amazonian catfish, I decided to set up a blackwater tank, which I’d then subject to seasonal variations in conductivity, pH, temperature, food accessibility and flow.

The idea was that imitating the seasonal floods of the forests from which the tetras came, would trigger spawning.


* 30 liter (about 8 gallon) tank

* A block of unprocessed, low-humic (not degraded), peat.

* Access to a water distillery (a reverse osmosis, a.k.a. RO, unit will do).

* Plants (mainly java moss, Pistia and Salvinia), oak leaves, and a hollow piece of driftwood.

* Air pump, heater, and a small circulation filter (360 liters per hour).

* Lighting: 11 watt compact fluorescent.


Figure 1. First I cut a 4 cm (1.5″) thick slice out of the peat block.


Figure 2. The peat was “low humic”, meaning the peat moss it is made of is not severely degraded, but very fibrous, and that pieces of the plant are still clearly identifiable. I chose this because I hoped it would hold together better in water than fine-particle high-humic peat – and it did!


Figure 3. The tank. An old battered 30 liter breeder. The slice of peat has been pressed into place, and water added. The peat holds together so well that even fresh after filling water there’s not much debris in the water. Most of what there was floated and could easily be scooped out.


Figure 6. Finished result. The layer of leaves turned out to be highly appreciated by the sailfins.



As it turned out, the tank functioned extremely well. The pH varied between 4.5 – 5.8, and I did get the sailfins to breed. The low pH meant the leaves degraded extremely slowly. The plants grew like crazy, especially the Javamoss, which almost took over the tank. The peat still held together perfectly, even after a year, and still colored the water, but no longer appreciably affected pH (which towards the end had drifted over 6.5).

But all wasn’t great. Downsides of using peat was that the water was very strongly colored, at its peak we’re talking more like coffee than tea! Also “dust” from the peat would clog the filter requiring frequent cleaning, and the lower parts of the peat turned anoxic, so when I removed it… well, let’s just say it didn’t smell like roses. For more details on how this tank operated, see my Breeding Sailfin Tetras article.

For a breeding tank intended to operate for a year or so, I think this set up worked very well indeed, but I’m not sure I’d use this approach in a more permanent aquarium.


ADDENDUM: Ron Last has used a very similar set-up and had it running for four years, and mailed this extremely interesting (Swedish language) link to me, where he details his experiences:




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