Red-tailed black shark and Siamese algae eater
Can these two share a tank? The assessment below is derived from stored behavioural and environmental traits, not from a compatibility chart.
Usually works. Minor points to watch are listed below.
- Minimum volume
- 270 L
- Minimum length
- 165 cm
- Factors considered
- 6
Concerns
Red-tailed black shark defends a territory and both animals use the same part of the tank, so give the territory-holder a defensible area away from the main swimming space.
Derived from: traits.territoriality and zone on both records
Both species occupy the bottom of the tank, so allow for the shared space when judging the footprint.
Derived from: zone and traits.activity on both records
Why it can work
- Temperature requirements overlap comfortably at 23–27 °C, with room for both animals to sit inside their preferred range.
- pH requirements overlap at 6.5–7.8.
- Hardness requirements overlap at 5–15 °dGH.
- Siamese algae eater needs a group of at least 4. Kept in smaller numbers it is not simply lonely — shoaling species held below their threshold show chronic stress behaviour, colour loss and, in several species, increased aggression towards tankmates.
Conditions
Most interesting pairings are conditional on something. These are the things the keeper can actually change.
- At least 4 Siamese algae eater, counted as one group
Derived, or documented?
These are different claims and are shown separately. The verdict above is derived from stored traits — arithmetic on AquariumHQ's own records. Whether the pairing is actually reported to work is a separate question, and for most pairs the honest answer is that we hold nothing.
Environmental compatibility
Derived from stored traits
Temperature, pH, hardness, size and behavioural traits compared between the two records. Sound as far as it goes, and it is not evidence that the pairing works.
Behavioural evidence
Widely reported in practice
Reported consistently across aquarium practice without a single citable source. Treated as real but not as measurement.
AquariumHQ's overall position
Conditional, and the condition is length rather than volume. In a long heavily broken-up tank this works often enough to be worth trying; in a standard community tank the algae eater is harassed continuously and the damage is behavioural long before it is visible.
What would change it: A tank long enough for two separate floor territories, hardscape that breaks the sight lines along it, and introducing the algae eater first so the shark arrives into an occupied tank rather than the reverse.
Factor by factor
- Shape recognition rather than species recognitionReported
The red-tailed black shark drives off fish with a similar outline, and the Siamese algae eater has one. Reports are consistent enough that this is the standard warning against the pairing, and the mechanism is what the shark treats as a rival rather than what it is.
- Shared floor territoryDerived
Both work the substrate and the lower surfaces of wood, so their claimed areas coincide almost exactly. There is no vertical separation available to reduce the overlap.
- Tank lengthDerived
The shark patrols a route. In a tank shorter than about 120 cm the route is the whole floor and there is nowhere the algae eater can be that is outside it.
| Depends on | How much |
|---|---|
| Tank size and dimensions | decisive |
| Life stage | significant |
| The individual animal | significant |
How good is the overlap?
Whether two ranges intersect is a necessary condition, not evidence. What matters is WHERE the shared band sits: an overlap at the top of one animal's range and the bottom of the other's is a forced compromise however wide it looks, and it is the case a simple intersection test cannot see.
| Dimension | Assessment | What that means here |
|---|---|---|
| Temperature | Comfortable for both | The shared band 23–27 °C sits centrally in both ranges, with room to drift. |
| pH | Comfortable for both | The shared band 6.5–7.8 pH sits centrally in both ranges, with room to drift. |
| Hardness | Comfortable for both | The shared band 5–15 °dGH sits centrally in both ranges, with room to drift. |
Factor by factor
Every factor states what it rests on. Most are derived from the traits stored on both records — sound arithmetic on AquariumHQ's own data, and not a report about how the pairing actually goes. Where a factor says 'not known', the data is missing and saying so matters, because a missing factor and a favourable one look identical once both are absent.
| Factor | Basis | Finding |
|---|---|---|
| Temperature | Derived from stored traits | Comfortable for both. The shared band 23–27 °C sits centrally in both ranges, with room to drift. |
| pH | Derived from stored traits | Comfortable for both. The shared band 6.5–7.8 pH sits centrally in both ranges, with room to drift. |
| Hardness | Derived from stored traits | Comfortable for both. The shared band 5–15 °dGH sits centrally in both ranges, with room to drift. |
| Body size | Derived from stored traits | Adult sizes are comparable at about 15 cm and 16 cm, so neither is a mouthful for the other. |
| Predation | Derived from stored traits | Highest predation risk in either direction is "none". |
| Territoriality | Derived from stored traits | Strongest territoriality across the pair is "strong". |
| Breeding aggression | Derived from stored traits | Neither becomes notably more aggressive while spawning. |
| Fin-nipping | Derived from stored traits | No nipper-and-target combination between these two. |
| Aggression | Derived from stored traits | Strongest intraspecific aggression across the pair is "strong". It bears on the pairing because a fish under pressure from its own kind redirects. |
| Social requirements | Derived from stored traits | Siamese algae eater needs at least 4. A shoaling species held below its threshold shows chronic stress and, in several species, increased aggression towards tankmates. |
| Group-size dependency | Derived from stored traits | The outcome depends on group size as much as on the pairing: under-grouped shoalers nip more and hide more. |
| Tank zone | Derived from stored traits | Both occupy the bottom of the tank, so the shared space is under pressure. |
| Activity level | Derived from stored traits | Activity levels are compatible. |
| Food competition | Derived from stored traits | Different feeding strategies, so direct competition is limited. |
| Risk to shrimp and snails | Derived from stored traits | Highest risk either animal poses to dwarf shrimp or snails is "high". |
| Effect on plants | Derived from stored traits | Neither animal damages planting. |
| Tank dimensions | Derived from stored traits | The more demanding of the two needs at least 180 L and 120 cm of length before anything else is in the tank. Volume alone does not describe a home for a fish that swims in straight lines. |
| Life stage | Evidence-backed | Life-stage dependency: significant. See the documented record. |
| Individual variation | Evidence-backed | Individual variation: significant. |
Environmental overlap
| Parameter | Shared range | Reading |
|---|---|---|
| Temperature | 23–27 °C | Aim for the middle of the shared band. |
| pH | 6.5–7.8 | Stability matters more than the exact figure. |
| Hardness | 5–15 °dGH | Check your source water before committing. |
What this assessment cannot tell you
- Individual temperament varies, particularly in cichlids and in bettas, and can override a favourable verdict for Red-tailed black shark or Siamese algae eater.
- The assessment is for the pair in isolation. A third species can change the outcome in either direction.
- Both animals are assessed at adult size, not at the size they are sold.
- A verdict is a statement about probability, not a guarantee.
Where the two water bands meet
Derived from the ranges AquariumHQ publishes for each species. The compatibility engine reads the same numbers; this is what they look like drawn on one axis.
Temperature (°C)
Scale: 0 to 36 °C.
pH (pH)
Scale: 4 to 9.5 pH.
Hardness (°dGH)
Scale: 0 to 30 °dGH.
Why the bands sit where they do
The overlap above is arithmetic. This is where the two numbers came from — the kind of water each animal is recorded from, and how much room that leaves.
Both animals come from hill stream and rapids, which is why the bands below sit where they do.
Red-tailed black shark
Hill stream and rapids
50–300 µS/cm · high flow · near saturation oxygen
Siamese algae eater
Hill stream and rapids
50–300 µS/cm · high flow · near saturation oxygen
- TemperatureRed-tailed black shark 22–27°CSiamese algae eater 23–28°C
Both inside their range from 23–27°C — 80% of the narrower animal’s range.
A wide window. Both animals are inside their own range across most of it, and the tank can be run without either sitting at an edge.
- pHRed-tailed black shark 6.5–7.8Siamese algae eater 6.5–8
Both inside their range from 6.5–7.8 — 100% of the narrower animal’s range.
A wide window. Both animals are inside their own range across most of it, and the tank can be run without either sitting at an edge.
- General hardnessRed-tailed black shark 5–15 dGHSiamese algae eater 4–18 dGH
Both inside their range from 5–15 dGH — 100% of the narrower animal’s range.
A wide window. Both animals are inside their own range across most of it, and the tank can be run without either sitting at an edge.
Computed from the ranges AquariumHQ publishes for each species, each of which records what it rests on — a source value, a habitat inference, a physiological requirement or a trade convention. Most are habitat inferences, and the species pages say which.