Veiltails and long fins: what filter flow really asks of these fish
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
Filters are chosen by turnover rate, rarely with a thought for the shape of the fish that will live in the flow. A study by Wakamatsu, Ogino and Hirata, published in 2019 in Scientific Reports, gives a direct measure of what long fins cost (see the study). Our equipment guides sit in the aquarium accessories section.
Contents · 5 sections
A shorter tail fin, a faster swimmer
The authors measured critical swimming speed — the fastest current a fish sustains before being swept back — in zebrafish from a long-finned strain, progressively trimming the caudal fin. With an intact fin of 14 to 30 mm: 23.4 cm/s. Trimmed to 10 mm: 26.2 cm/s. At 5 mm: 27.1 cm/s, roughly 16% better. The curve then reverses, at 3 mm (25.2) and 1 mm (21.7): there is an optimum, around 3 to 10 mm, and too short a fin handicaps as much as too long a one.
Temperature weighs at least as much
The same study shows a performance plateau between 16 and 30 °C, with critical speed steady around 22 to 24 cm/s. Outside it, the drop is steep: 11.6 cm/s at 12 °C and 15.9 cm/s at 36 °C. A tank held at 34 °C already costs nearly a quarter of swimming capacity. That applies to every fish in the tank, not just the fancy-finned ones.
Current is not wanted for its own sake
DePasquale, Fettrow, Sturgill and Braithwaite tested in 2019, in Applied Animal Behaviour Science, what zebrafish choose when given the option (see the study). A zone that was both structured and flowing scored a clearly positive preference index; flow alone, without structure, was avoided, as was a bare zone. Fish do not flee current: they flee current with nowhere to shelter. Our article on aquarium enrichment covers what those structures do besides.
What these measurements cannot be made to say
They concern Danio rerio and nothing else. Transferring the centimetres per second to a betta, a veiltail guppy or an oranda goldfish would be unwarranted: body shape, swimming style and size all differ. The fins were cut, which does not exactly reproduce a naturally short tail. And critical speed measures peak performance, not a fish's day-to-day comfort. One technical caveat: the 2019 preference paper was the subject of a 2023 corrigendum concerning numerical values — only the preference indices are used here.
- +16% critical speed going from a 14-30 mm tail fin to 5 mm, in zebrafish.
- Optimum between 3 and 10 mm: too short penalises too.
- 11.6 cm/s at 12 °C against 22-24 cm/s across the 16-30 °C range.
- Current is only chosen when paired with structures to shelter behind.
The practical consequence comes down to two adjustments rather than a flow calculation. First, aim the filter outlet at a pane or at hardscape rather than down the length of the tank, to break the flow — equipment choice is covered in our guide to the internal or external filter. Second, leave calm zones behind plants or rocks where a long-finned fish can settle. A veiltail permanently pinned into a corner with clamped fins is a fish that is working — and fins damaged by that effort open the door to the problems described in our article on betta fin rot.
Frequently asked questions
Does a betta need a weaker filter?
Broken flow rather than an undersized filter: biological filtration depends on turnover. Aiming the outlet at a wall and creating sheltered zones answers both needs.
Do the zebrafish figures apply to my veiltail goldfish?
The principle, yes: a very elongated tail fin costs swimming capacity. The numbers, no — they were established in zebrafish only.
My fish sit in the current — is that a bad sign?
Not if calm zones exist alongside. The problem is the absence of an alternative, not the current itself.
An accessory from our shop for this use: Stick-on digital thermometer for aquarium and terrarium glass (€18.90) · the whole shop
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.
