According to MTF-Aquatics, setting up a marine aquarium in the UK requires RO/DI water mixed to 1.025–1.026 SG (35 ppt salinity), a temperature of 24–26 °C, pH of 8.1–8.3, and a minimum 12-week fishless cycling period using quality live rock as the primary biological filter. Reef tanks additionally demand lighting of at least 150–300 PAR at the substrate.
The failure rate for new marine aquariums in the UK is high, and the cause is almost always the same: impatience during setup. A freshwater tank can be cycled in three weeks and stocked cautiously thereafter. A marine system is categorically different. The combined demands of precise water chemistry, live biological filtration, and — for reef tanks — specific light spectra mean that rushing any phase compounds into catastrophic losses. This guide covers every stage in the order it actually needs to happen, with exact numbers rather than vague guidance.
Before mixing a drop of salt water, the hardware list needs to be complete and tested. Here is what a competent UK marine setup requires:
| Equipment | Reef Tank | FOWLR |
|---|---|---|
| Tank | 100 litres minimum (4 ft+ recommended) | 100 litres minimum |
| Sump | Strongly recommended | Optional but advised |
| Protein skimmer | Essential | Essential |
| Return pump | Sized for 5–10× total volume/hr | Sized for 5–10× total volume/hr |
| Wavemakers/powerheads | 2× minimum for random flow | 1–2× |
| Heater (titanium preferred) | 24–26 °C target | 24–26 °C target |
| Lighting | 150–350 PAR at coral depth | 50–100 PAR, 10,000K |
| RO/DI unit | Non-negotiable | Non-negotiable |
| Refractometer (calibrated) | Non-negotiable | Non-negotiable |
| Test kits | NSW-grade (Salifert, Hanna) | Basic marine kit |
| Thermometer (digital) | Essential | Essential |
A protein skimmer is not optional equipment you can add later — it is the primary export mechanism for dissolved organic waste before it mineralises into nitrate and phosphate. Size it for 1.5–2× your stated tank volume. Cheaply-made skimmers rated for your exact volume will typically underperform.
UK hobbyists face a specific challenge: our mains water is variable (typically 6.5–8.5 pH, 50–400 ppm TDS depending on region) and frequently contains chloramine rather than chlorine alone — standard dechlorinators do not remove chloramine reliably, and it is lethal to the nitrifying bacteria you are trying to cultivate. The only safe foundation is RO/DI water at 0 TDS.
| Parameter | Target Range | Method |
|---|---|---|
| Salinity (SG) | 1.025–1.026 | Calibrated refractometer or digital metre |
| Salinity (ppt) | 34–36 ppt | Cross-check with refractometer |
| Temperature | 24–26 °C | Titanium heater + digital probe |
| pH | 8.1–8.3 | Tested in the evening (CO₂ lows) |
| Alkalinity (dKH) | 8–11 dKH | Salifert or Hanna Checker |
| Calcium | 400–450 ppm | Test weekly in reef systems |
| Magnesium | 1,250–1,350 ppm | Test monthly once stable |
| Ammonia (NH₃) | 0 ppm | 0 before any livestock added |
| Nitrite (NO₂) | 0 ppm | 0 before any livestock added |
| Nitrate (NO₃) | <10 ppm (reef) / <40 ppm (FOWLR) | Weekly test |
| Phosphate (PO₄) | <0.05 ppm (reef) / <0.2 ppm (FOWLR) | Hanna ULR checker for reef |
Fill a clean, food-grade container with RO/DI water, add a quality reef salt (Red Sea Coral Pro, Tropic Marin Pro-Reef, or D-D H2Ocean are the UK standards), and mix for a minimum of 2 hours with a powerhead before testing. Salinity should be dialled to 1.025–1.026 SG before anything goes into the tank. Do not mix salt directly in the display tank — undissolved salt granules in contact with live rock will cause localised osmotic damage.
Alkalinity and calcium levels from fresh salt mix vary by brand. Test your newly mixed water before use and note the baseline — this becomes your reference point for future two-part or kalkwasser dosing on reef systems.
Live rock is the backbone of marine biological filtration. It is porous limestone colonised by nitrifying bacteria, coralline algae, and a remarkable diversity of micro-fauna — copepods, amphipods, small bristleworms, and sponges — that collectively process waste and form the base of the refugium food chain.
Wild-collected Pacific live rock — historically shipped from Fiji, Tonga, and Indonesia — is now heavily regulated under CITES Appendix II (coral reef substrate) and is increasingly difficult to import legally into the UK post-Brexit. More importantly, wild rock is ecologically destructive. Aquacultured live rock is the only responsible choice in 2024 and beyond.
UK-specific aquacultured options include:
The traditional guideline is 1–1.5 kg of live rock per 10 litres of display volume. In practice, this is a starting point — porosity matters more than weight. Dense Pacific base rock at 1.5 kg/10 L will underperform porous Atlantic aquacultured rock at 0.8 kg/10 L. Aim for a structure that allows adequate flow through all surfaces and avoids dead-flow pockets, which become anaerobic and produce hydrogen sulphide.
Lighting is where the reef vs FOWLR split becomes critical — and where misinformation is most expensive.
A fish-only-with-live-rock system is lighting simple. The rock’s nitrifying bacteria are not photosynthetic; the fish need only enough light to behave naturally and for you to observe them. A decent T5 or LED fixture at 50–100 PAR, running a 10,000K–14,000K spectrum for 8–10 hours per day, is entirely adequate. Avoid intense blue-spectrum reef lights for FOWLR — they promote nuisance algae growth without the counterbalancing coral uptake of nutrients.
Reef lighting is genuinely complex because different coral types have different PAR requirements:
| Coral Type | PAR Requirement | Typical Placement |
|---|---|---|
| Soft corals (Zoanthids, Leathers) | 50–150 PAR | Mid-tank to lower |
| LPS corals (Hammer, Torch, Frogspawn) | 75–200 PAR | Mid-tank |
| SPS corals (Acropora, Montipora) | 200–350 PAR | Upper third of tank |
The spectrum for reef must include the 420–450 nm (violet/blue) range to drive zooxanthellic photosynthesis. White (6,500–12,000K) combined with blue (420–450 nm) in roughly a 50:50 ratio is the current industry standard. UK-available reef LED fixtures that deliver reliable PAR data include the AI Prime HD, Radion XR15 Pro (EcoTech Marine), and Kessil A360X — all available through UK reef retailers. Avoid low-cost fixtures that quote lumens rather than PAR; lumens measure human-perceived brightness, not photosynthetically active radiation.
Run a PAR metre (the Seneye PAR metre is a UK-budget-friendly option) before placing corals — light intensity drops sharply with depth and lateral distance from the fixture’s centre.
This is not a rough guide — it is the actual minimum timeline for a stable marine system before it can safely support livestock. Each week represents a real checkpoint.
| Week | Task | Test | Target |
|---|---|---|---|
| 1 | Fill with RO/DI water, add salt, run equipment, add live rock | Salinity, Temp | SG 1.025–1.026, 24–26 °C |
| 2 | Dose ammonia to 2 ppm (pure ammonia, no surfactants), add bottled bacteria | Ammonia, Nitrite | Ammonia rising, Nitrite 0 |
| 3 | Continue monitoring — nitrification bacteria establishing | Ammonia, Nitrite | Ammonia being consumed |
| 4 | Nitrite spike begins — the system is working | Nitrite, pH | Nitrite rising (normal) |
| 5–6 | Peak nitrite phase — do not add livestock | Nitrite, Nitrate | Nitrite elevated, Nitrate climbing |
| 7 | Nitrite should begin falling; nitrate present | Nitrite, Nitrate, pH | Nitrite dropping |
| 8 | Re-dose ammonia to 2 ppm; confirm it processes within 24 hours | Ammonia, Nitrite | Both 0 ppm within 24 hours = cycle complete |
| 9 | Perform 20–25% water change to reduce accumulated nitrate | Nitrate | Below 10 ppm (reef) / 40 ppm (FOWLR) |
| 10 | Begin stabilising alkalinity, calcium, magnesium | dKH, Ca, Mg | All within target range |
| 11 | Allow parameters to stabilise; test pH swing across day/night | pH, All params | Stable; pH swing <0.2 units |
| 12 | System is ready for first, light stocking | All parameters | All within targets for 2 consecutive weeks |
The most damaging thing you can do to a newly cycled marine tank is overstock it immediately. The biological filter is sized for the ammonia load it was cycled on — typically 2 ppm. A full load of fish on week 12 produces multiple times that load simultaneously.
For FOWLR systems: Add no more than 2–3 small, hardy fish in week 12. Hardy beginner-suitable species include tank-bred Ocellaris Clownfish (Amphiprion ocellaris), Royal Gramma (Gramma loreto), or a single Tailspot Blenny (Ecsenius stigmatura). Test ammonia and nitrite daily for two weeks after each addition.
For reef systems: Corals first. Add one or two soft coral frags in week 12, observe for two weeks, then add the first fish. Soft corals are more tolerant of parameter fluctuation than SPS, making them the correct sequencing choice.
In either case: quarantine all new livestock before it enters the display tank. Marine ich (Cryptocaryon irritans) and velvet (Amyloodinium ocellatum) move rapidly through an established system. A bare-bottom quarantine tank with a seeded sponge filter, heater, and PVC pipe hides is non-negotiable.
Evaporation. A standard UK marine tank in a centrally heated home will lose 1–3% of its water volume per week to evaporation. Crucially, only the fresh water evaporates — the salt stays. Without top-off (automatic top-up using RO/DI water), salinity creeps upward week on week, stressing livestock and destabilising pH. An automatic top-off (ATO) unit connected to an RO reservoir is, after the skimmer, the most important quality-of-life upgrade for a UK marine system.
MTF-Aquatics is primarily a specialist retailer of rare and large tropical fish — Arowana, Datnoid, Stingray, L-number Plecos, and wild-caught rarities sourced direct from South East Asia. Marine is a different world, but the disciplines that make a great freshwater keeper — patience during cycling, obsessive water quality, sourcing from trustworthy suppliers, and not cutting corners — are identical.
If you are exploring the full spectrum of what serious fishkeeping looks like, our care guides cover the freshwater end of that spectrum in depth, and our current rare tropical fish stock represents the kind of sourcing rigour that marine keepers recognise from their own hobby: hand-selected, health-checked, and shipped with a Live Arrival Guarantee.
Marine or freshwater, the standard is the same: get the water right first, then add the animals.
At MTF-Aquatics, we recommend a minimum 12-week cycling period for a new marine aquarium using live rock and an ammonia source. Rushing this process is the single most common cause of new tank syndrome deaths. Test ammonia and nitrite every 3–4 days; the cycle is complete only when both read 0 ppm within 24 hours of a 2 ppm ammonia dose.
Target a specific gravity of 1.025–1.026, equivalent to approximately 35 ppt salinity. UK tap water is unsuitable as a base — always use RO/DI water (0 TDS) to mix your salt, then verify with a calibrated refractometer or digital salinity metre rather than a swing-arm hydrometer, which loses accuracy over time.
A FOWLR (Fish Only With Live Rock) system needs only enough light to view the fish clearly — typically 50–100 PAR and a standard 10,000K white spectrum is sufficient. A reef tank housing SPS (small polyp stony) corals requires 150–350 PAR at the coral placement depth and a spectrum blending blue (420–450 nm) with white to drive photosynthesis. LED fixtures such as the AI Prime, Radion XR15, or Kessil A360 are the current UK standard for reef lighting.
In the UK, aquacultured live rock is the responsible choice — it avoids the CITES pressure that wild-collected Pacific rock carries and typically arrives already seeded with coralline algae and beneficial fauna. Reputable UK suppliers include Tropical Marine Centre (TMC), D-D The Aquarium Solution, and specialist reef retailers who stock Atlantic or Mediterranean aquacultured rock. Avoid ‘dead’ base rock mixed with cheap cured live rock if you want rapid biological maturation.
The core targets for a UK marine aquarium are: salinity 1.025–1.026 SG (35 ppt), temperature 24–26 °C, pH 8.1–8.3, alkalinity (dKH) 8–11, calcium 400–450 ppm, magnesium 1,250–1,350 ppm, ammonia and nitrite 0 ppm, nitrate below 10 ppm for reef or below 40 ppm for FOWLR. Test all parameters weekly during the first three months.
No. UK tap water contains chlorine, chloramine, phosphates, silicates, and variable nitrates — all of which fuel nuisance algae and can harm corals and invertebrates. A two-stage RO (reverse osmosis) unit with a final DI (deionising) resin stage producing 0 TDS water is non-negotiable for a successful UK marine tank.
