Seahorse Aquarium Guide: How to Keep Seahorses in Saltwater

A seahorse aquarium is a calm, tall saltwater tank built around the needs of slow-swimming seahorses: at least 110–120 litres and 45 cm deep for a pair, gentle flow, plenty of hitching posts, 23–26 °C for tropical species and frozen mysis two or three times a day. Always choose captive-bred seahorses, never wild-caught.

This guide is for UK hobbyists planning a seahorse tank, from first-timers to experienced reefers. It answers whether seahorses are saltwater fish, compares the species you can buy, and covers tank size, temperature, flow, feeding, tankmates, disease, UK law and CITES rules, plus where natural seawater fits in.

Updated October 2026 by the team at Natural Seawater Supplier, a Croydon, London-based supplier of live natural seawater for marine aquariums, delivering across the UK 7 days a week.

Key takeaways

• All true seahorses (genus Hippocampus) are saltwater fish; a few tolerate brackish water, but none lives in freshwater.
• A pair of standard seahorses needs at least 110–120 litres and 45 cm of water depth, plus about 40 litres for each additional pair.
• Feed captive-bred seahorses thawed frozen mysis shrimp two or three times a day, ideally at a fixed feeding station.
• All seahorse species have been listed on CITES Appendix II since May 2004, so legal trade needs CITES documentation.
• The UK’s two native seahorses, Hippocampus hippocampus and Hippocampus guttulatus, have been protected under the Wildlife and Countryside Act 1981 since April 2008 and must not be collected.

Are seahorses saltwater or freshwater?

Seahorses are saltwater fish. Every one of the roughly 46 recognised species in the genus Hippocampus is marine, living in seagrass beds, mangroves, reefs and estuaries from the tropics to temperate seas such as the waters around Britain.

A few species, including the common or yellow seahorse (H. kuda) and the slender seahorse (H. reidi), regularly enter brackish estuaries and can cope with reduced salinity for a while. That is not the same as being a freshwater fish. If a shop sells a “freshwater seahorse”, it is almost certainly a freshwater pipefish, a related but different animal.

In practice, a seahorse fish tank should run at normal marine salinity: 34–35 ppt, or a specific gravity of 1.025–1.026.

Which seahorse species can you keep?

Choose a captive-bred species and match the temperature to it. Never mix tropical and temperate species.

Common name	Scientific name	Adult size (cm)	Temperature	Suitability
Lined seahorse	Hippocampus erectus	19	21–25 °C	Best for beginners; hardy, widely captive-bred
Slender (longsnout) seahorse	Hippocampus reidi	17	23–26 °C	Good; colourful, captive-bred
Common / yellow seahorse	Hippocampus kuda	17	24–26 °C	Moderate; captive-bred lines available
Tiger tail seahorse	Hippocampus comes	18	24–26 °C	Moderate; captive-bred lines available
Zebra-snout (Barbour’s) seahorse	Hippocampus barbouri	15	24–26 °C	Intermediate; needs very clean water
Dwarf seahorse	Hippocampus zosterae	2.5	22–26 °C	Specialist; needs live baby brine shrimp daily
Big-belly seahorse	Hippocampus abdominalis	35	16–20 °C	Temperate; needs a chiller and large tank
Short-snouted seahorse (UK native)	Hippocampus hippocampus	15	Temperate	Protected in the UK; not for aquariums
Spiny seahorse (UK native)	Hippocampus guttulatus	18	Temperate	Protected in the UK; not for aquariums

The lined seahorse is the classic first choice: it is tolerant of a wide temperature range, eats frozen food readily and is bred in large numbers. See its FishBase species profile for natural range and biology.

Seahorse tank size and shape

Seahorses swim slowly and vertically, and pairs rise up through the water together during courtship. Depth is therefore more important than length.

Seahorses	Minimum volume	Minimum water depth
1 pair of standard species (H. erectus, H. reidi, H. kuda)	110–120 litres	45 cm (ideally 50–60 cm)
Each additional pair	+40 litres	Same
Dwarf seahorses (H. zosterae), small group	20–40 litres	25–30 cm
Big-belly seahorses (H. abdominalis), 1 pair	250 litres+	60 cm+

A tall cube or column-style tank of 45–60 cm depth makes a good seahorse tank. Avoid shallow, wide “reef” tanks and avoid open-top designs if you use powerful lighting that heats the surface.

Seahorse tank setup: flow, hitching posts and filtration

Water flow

Seahorses are weak swimmers with less efficient gills than most fish. Give them gentle, diffuse flow (roughly 5–10 times tank volume per hour, spread out, never a direct jet) and a few calm zones. Cover every pump intake and powerhead with a foam guard; seahorses will grab onto anything and can be sucked against an intake or injured.

Hitching posts

Seahorses rest by wrapping their tails around a holdfast. Provide at least two hitching posts per seahorse at different heights. Good options include artificial gorgonians, soft silicone plants, mangrove roots, macroalgae and thin branches of live rock. Avoid sharp or rough décor that can scratch their skin.

Filtration

Seahorses are messy, frequent feeders, so filtration must be oversized: live rock or a matured biological filter, a protein skimmer rated for a larger tank, and mechanical filtration you clean weekly. An ultraviolet (UV) steriliser is widely used in seahorse systems to reduce free-floating bacteria.

Lighting

Moderate lighting is enough unless you keep corals. Seahorses prefer a regular day-night cycle of around 10–12 hours; our guide to reef tank lighting schedules explains how to set one.

Water parameters for a seahorse aquarium

Parameter	Target
Temperature (tropical species)	23–26 °C (many keepers use 23–24 °C to slow bacterial growth)
Temperature (H. erectus)	21–25 °C
Salinity	34–35 ppt (SG 1.025–1.026)
pH	8.0–8.3
Ammonia / nitrite	0
Nitrate	Below 10 ppm
Phosphate	Below 0.1 ppm

Change 10–20% of the water every week and siphon uneaten food from the feeding area after every meal. Our guides on ideal marine tank water parameters and how to do a saltwater water change step by step cover testing and technique.

Feeding seahorses

Seahorses have no stomach and digest food quickly, so they need to eat often. Captive-bred seahorses are trained to eat frozen food, which is the main reason they are so much easier to keep than wild-caught ones.

1. Feed thawed frozen mysis shrimp two or three times a day (most keepers aim for three).
2. Thaw in a small cup of tank water, rinse, and ideally soak for 15–20 minutes in a vitamin or fatty-acid enrichment.
3. Offer food in a fixed feeding station, such as a small dish or clam shell in a low-flow area, so seahorses learn where to find it.
4. Feed as much as they will eat in about five minutes.
5. Remove leftovers after 30 minutes; rotting mysis is the commonest cause of water-quality problems in seahorse tanks.

Dwarf seahorses are different: they need live, newly hatched brine shrimp (Artemia nauplii) several times a day. For general feeding principles, see our guide on how often to feed saltwater fish.

Seahorse tankmates and corals

Seahorses are slow, deliberate feeders, so tankmates must be peaceful and slow too.

Good tankmates	Avoid
Other seahorses of the same temperature range	Clownfish, damselfish, tangs, triggerfish, wrasse and other fast feeders
Small gobies, firefish, Banggai and pyjama cardinalfish	Anemones (they sting and can eat seahorses)
Nassarius, cerith and turbo snails	Large crabs and big hermit crabs
Small, reef-safe hermit crabs in moderation	Lionfish, eels and any fish big enough to swallow a seahorse
Non-stinging corals such as mushrooms, leathers and soft macroalgae	Stinging LPS (torch, hammer, frogspawn) and fire coral

For the clean-up crew, choose small, peaceful species: our guide to reef-safe hermit crabs lists the dwarf species least likely to bother seahorses.

Common seahorse diseases

Bacterial infections (Vibrio)

Seahorses have a thin layer of skin over bony plates rather than scales, which makes them vulnerable to bacterial infections, most often Vibrio species. Signs include white patches, tail rot, snout rot, loss of appetite and lethargy. Prevention is the best cure: keep the temperature in the lower part of the range, remove uneaten food promptly, avoid overstocking and consider a UV steriliser. Consult an aquatic vet for antibiotic treatment.

Gas bubble disease

Gas bubble disease shows as bubbles under the skin, on the tail or snout, or as an over-inflated brood pouch in males, leaving the seahorse floating. It is linked to water supersaturated with gas, for example from a pump drawing in air, and to bacterial infection. Check equipment for micro-bubbles, keep the tank deep enough for seahorses to use the full water column, and seek specialist advice before treating.

Quarantine

Quarantine new seahorses for at least four weeks in a separate tank, and never mix seahorses from different sources without quarantine, because each group may carry different bacteria.

UK law, CITES and buying live seahorses

CITES Appendix II

All seahorse species have been listed on CITES Appendix II since 15 May 2004. Trade is allowed but must be legal, sustainable and documented, so imports into the UK require CITES permits. The IUCN Seahorse, Pipefish and Seadragon Specialist Group explains how the listing works.

UK native seahorses are protected

Britain has two native species, the short-snouted seahorse (Hippocampus hippocampus) and the spiny seahorse (Hippocampus guttulatus). Both were added to Schedule 5 of the Wildlife and Countryside Act 1981 by The Wildlife and Countryside Act 1981 (Variation of Schedule 5) (England) Order 2008, which came into force on 6 April 2008. It is an offence to take, kill, possess or disturb them, so never collect a seahorse from British waters.

Where to buy live seahorses

Buy only captive-bred seahorses from a specialist breeder or retailer who can confirm the species, its origin and its CITES paperwork. Healthy seahorses are alert, have a full, rounded belly, clear skin, and are eating frozen mysis in front of you.

Natural seawater for seahorse tanks

Seahorses need a stable environment more than anything, so the water you add during water changes should match the tank exactly. Our natural seawater is collected, delivered ready to use and typically measures 34–36 ppt salinity, pH 8.0–8.3, ammonia and nitrite zero and nitrate 0–0.9 ppm, which suits a low-nitrate seahorse system. Many seahorse keepers run new water past a UV steriliser as standard practice, whichever water they use. You can compare it with mixing salt in natural seawater vs synthetic salt, or try 25 litres of natural seawater free. Setting up from scratch? Follow our reef tank setup guide and cycle the tank fully before adding seahorses.

Frequently asked questions

Are seahorses saltwater fish?

Yes. All true seahorses (genus Hippocampus) are marine fish that live in saltwater. A few species, such as Hippocampus kuda and Hippocampus reidi, tolerate brackish estuaries and mangroves, but none lives permanently in freshwater. A seahorse aquarium must be a saltwater aquarium.

Can seahorses live in freshwater?

No. Seahorses cannot survive long in freshwater. Fish sold as freshwater seahorses are usually freshwater pipefish, close relatives of seahorses that live in rivers. Keep true seahorses at a salinity of about 34–35 ppt.

How big does a seahorse tank need to be?

For a pair of standard-sized captive-bred seahorses such as Hippocampus erectus or Hippocampus reidi, use at least 110–120 litres and add about 40 litres for each extra pair. Height matters most: aim for at least 45 cm of water depth, ideally 50–60 cm. Dwarf seahorses can live in 20–40 litres.

Are seahorses hard to keep?

Captive-bred seahorses are not hard to keep if their tank is set up for them. They need a dedicated, calm aquarium with low flow, hitching posts, very clean water and frozen mysis two or three times a day. Wild-caught seahorses are much harder and should be avoided.

Can I buy live seahorses in the UK?

Yes. Captive-bred tropical seahorses are legally sold in the UK by specialist retailers, but all seahorses are on CITES Appendix II, so imports need CITES documentation. The UK’s native short-snouted and spiny seahorses are protected and must not be taken from the wild or kept.

Can seahorses live with clownfish?

It is not recommended. Clownfish are fast, territorial feeders that outcompete seahorses for food, and they often host anemones, which can sting and eat seahorses. Choose slow, peaceful tankmates such as small gobies, firefish and cardinalfish instead.

Sources

External sources cited in this article (checked October 2026):

• FishBase species profile (FishBase)
• IUCN Seahorse, Pipefish and Seadragon Specialist Group (IUCN SSC)
• The Wildlife and Countryside Act 1981 (Variation of Schedule 5) (England) Order 2008 (legislation.gov.uk)

Bottom line

Seahorses are saltwater fish, and captive-bred seahorses are a realistic project for a careful UK hobbyist. Give a pair at least 110–120 litres and 45 cm of depth, gentle flow, plenty of hitching posts and peaceful tankmates; keep tropical species at 23–26 °C; feed frozen mysis two or three times a day; and keep the water exceptionally clean. Buy captive-bred only, with CITES paperwork, and leave Britain’s protected native seahorses in the sea.