Some fish do not spawn at all without water movement
Anyone who has ever been diving or snorkelling in the sea has probably also experienced currents: either gentle, so that you hardly need to paddle, or so violent that you realise that it’s stronger than your fitness levels and you just have to surrender yourself to your fate. Even really dangerous downward currents spit you out again at some point - you really just have to stay calm and save your breath?
In freshwater, which we are talking about here, there are all kinds of currents: no current at all, as in a lake, very gentle currents - mostly in the lowlands and larger rivers - or also strong currents, mostly in mountainous regions. But hardly anyone dares to go into these very strong currents.
However if you want to examine the habitat of fish species in detail, you’ll need to pluck up your courage and take a look under the water surface. And you will be surprised! In contrast to the often pear-shaped bodies of humans, flow-loving fish are extremely adapted. They either stand in the current with little energy expenditure or hold on to the slippery substrate with suction organs or fins that have been converted into adhesive organs (e.g. gobies).
In Vietnam, we had a barbel specialist with us on a JBL expedition. He observed how barbels and danionins actively seek out sections with strong currents in a clear stream to snatch at plankton swimming by. After his return, he used flow pumps in his aquariums - just like marine aquarists do - for some species which had so far not been able to reproduce. Within two weeks, the animals spawned!
Can you measure flow?
After having experienced many different strong currents in freshwater, the question naturally arises: What is a strong, medium or gentle current? Can you measure current and check in the aquarium how much current is there? Yes, you can!
For our next expedition, I bought a measuring device from the Windaus company, whose propeller transmits the number of its revolutions to a display. The number of revolutions in one minute could then be converted into current speed via an enclosed table. The currents in the rivers and streams visited ranged from 1.4 to 11 km/h.
Okay, but how much current do we have in the aquarium? In our research department we set up an aquarium in which we tried out a wide variety of technology: external filters, internal filters, submersible pumps, with and without jet nozzles, etc. The results were really surprising: an internal filter with a pump capacity of 800 l/h produced a flow velocity of 1.4 km/h at a distance of 10 cm in front of its outlet. At a distance of 30 cm, this dropped to 1.3 km/h. Attaching a wide jet pipe reduced the flow somewhat and a spray bar eliminated any flow to zero - very sobering!
Then we introduced a large submersible pump, whose specifications claimed it had (a mighty) 2,000 l/h. At a distance of 10 and 30 cm, it achieved a speed of 2.16 km/h. At a distance of 80 cm, no flow was measurable. A swimming pool pump that we had brought along not only produced a wet room, but also fish that stuck to the glass (not really - only kidding).
Unlike filters, flow pumps are the only real way to create a targeted flow in the aquarium!
This issue of current is really exciting if a little complex! In an episode of "JBL TV" I explain it with examples/illustrations: JBL TV #14: The current in the aquarium. How much do you need? How important is the current?