Side by side
| What matters | Dosing pump (two-part) | Calcium reactor |
|---|---|---|
| Cost to start | A pump plus supplements; usually cheaper to start | Reactor, CO₂ regulator, media and often pH control; costs more to start |
| What it adds | Separate calcium and alkalinity solutions, plus magnesium or trace products if you need them | Calcium and carbonate from dissolved media; the exact output depends on tuning and media |
| How you control it | Calibrate the pump and adjust your daily or weekly doses | Tune the output flow, CO₂ bubble rate and pH inside the reactor |
| Growing with your tank | Add channels or bigger containers as demand grows | Efficient at high demand, but you have to find space for it and keep CO₂ supplied |
| What can go wrong | An empty container, a blocked line, calibration drift or a programming mistake | A CO₂ or pH control fault, a blocked outlet, spent media or a pump failure |
How the costs compare
Current UK prices show the gap clearly. A four-channel Red Sea ReefDose 4 is listed at £329, while a small Deltec PF509 calcium reactor is around £378.99, before you add CO₂ equipment and ongoing media. A Deltec Twin-Tech 1500 is listed at around £1,999.99. These are example prices, not a full forecast of what you’ll spend.
Which suits a smaller reef?
For a new or lightly stocked tank, a reactor is hard to justify before your corals have shown they need it for months on end. A doser running a cautious two-part routine keeps your chemistry easy to see and costs less to start. As demand grows, weigh the cost of bottles, pump channels and upkeep against a reactor’s CO₂ and media.
Neither one does your testing for you
Both still need regular calcium and alkalinity tests. A reactor can deliver a steady output while your tank’s real demand changes, and a doser can pump its programmed amount while a line is blocked. Testing is how you catch either problem.
Sources and limits
We checked current UK prices for the Red Sea ReefDose 4, Deltec PF509 and Deltec Twin-Tech 1500. We haven’t measured capacity, stability, noise or running costs ourselves.
THE SHORT ANSWER
Dosing pumps are easier to control; reactors suit high, steady demand
A dosing pump adds measured amounts of ready-made liquids, so it’s fairly simple to start, adjust and understand. The cost of solution and the size of the containers grow as your tank uses more. A calcium reactor uses CO₂ to dissolve media and supply calcium and alkalinity continuously. That can suit large, hungry systems, but it means a reactor, regulator, CO₂ cylinder, feed pump, pH management and a lot more tuning.
For most new or moderately stocked reefs, dosing is the easier route: you can raise it gradually and see exactly what’s going in. A reactor makes more sense once demand is high and steady enough that the cost of solutions, mixing and refilling dominates. Compare the full first-year cost, the space each needs and what happens when each one fails.
STEP BY STEP
How to do it
- 01
Measure your daily demand
Work it out from repeated alkalinity tests and your estimated water volume. Without that number, you can’t size or cost either option honestly.
- 02
Cost the dosing route
Work out the solution volume, pump channels, containers, tubing, a year’s supplements and tests. Include anything needed to keep salinity in check.
- 03
Cost the reactor route
Price the reactor, regulator, cylinder, media, feed pump, any probe or controller, refills, and safe space in your cabinet.
- 04
Compare what happens when things fail
Write down what happens after a blocked line, a stuck pump, an empty container, a CO₂ fault, a pH swing or a power cut, and how you’d notice each one.
YOUR SITUATION
What to do in your situation
The volume of solution and how often you refill are manageable
A dosing pump is usually the clearer option, and easier to undo.
Large daily volumes and their cost dominate
A calcium reactor can pay for itself, if you’re experienced and monitor it closely.
A cylinder or containers won’t fit safely
Choose the option where everything fits, not just the main unit.
COMMON MISTAKES
Mistakes to avoid
Comparing a pump’s price with a reactor’s
A fair comparison includes containers and solutions on one side, and the regulator, cylinder, probe and media on the other.
Buying for demand you don’t have yet
Going big too early adds complexity before your tank has shown it needs it.
Assuming a pH controller makes it safe
A controller can shut things off or raise an alert, but it can’t make up for poor tuning, bad calibration or the wrong setup.
WORKED EXAMPLE
When solution volume changes the answer
A mature SPS tank uses so much alkalinity that the two-part containers need mixing constantly and fill most of the cabinet. The yearly cost of supplements and the time spent refilling now add up, and demand has held steady for months.
The owner compares a complete reactor setup, plus easy access to CO₂ refills, with twelve months of current dosing costs. If it pays back, the cabinet is ventilated and there’s a monitoring plan, the reactor may be worth it. If demand is still climbing unpredictably, or the cylinder can’t be handled safely, dosing stays the easier option to control.
BEFORE YOU CHANGE ANYTHING
Write down where you’re starting from
Note the date, the equipment or product you’re using, how you measured and the conditions behind every important number. Say whether a volume is the tank’s label figure or your real net water; whether a flow is the box rating or what actually comes out; and, for test results, the salinity, test kit and time of day. A precise-looking number is useless once you’ve forgotten where it came from.
Change one thing at a time, and decide in advance what improvement will look like: a steady test trend, less detritus, a more stable temperature, a quieter tank or a lower running cost. One reading, or one good day, isn’t a trend, and a setting copied from someone else’s tank isn’t proof it suits yours. The free Reef Companion keeps your tests, maintenance and changes in one dated log, so you can see cause and effect.
MAKING CHANGES SAFELY
Protect your tank while you make the change
Take a baseline
Measure before you install equipment, change your chemistry or alter your routine. Take photos if placement matters, and note how your livestock behaves. Without a starting point, you can’t be sure what caused any difference later.
Start small
Begin with a cautious setting or dose, and keep a way back. Hang on to the old equipment until the new one has proved itself, keep your previous schedule, and don’t add livestock at the same time as a big lighting, flow or chemistry change.
Give it the right amount of time
Mechanical problems show up straight away; nutrient and coral responses can take days or weeks. Judge the change over a sensible period, but stop immediately if you see animals in distress, a leak, overheating or an electrical risk.
Afterwards, note the result and decide whether to keep, adjust or undo the change. If the result is unclear, go back to your last stable setup and measure better, rather than piling on another change.
SIX MONTHS ON
Check it’s still the right decision
What made sense on day one can stop working as livestock grows, the room warms up, equipment gets dirty or a routine turns out to be unrealistic. Set a reminder to review it in six months, comparing what you assumed with the reality: water volume, electricity, consumables, noise, cleaning time and test trends.
Test trends, real flow, temperature, light coverage or whatever the change was for
Is it still doing its job, with room to spare?
How often you clean it, how hard it is to reach, calibration drift and missed tasks
Can you keep this up without cutting corners?
Electricity, reagents, filters, spare parts, warranty and downtime
Would you make the same choice knowing the real cost?
If it’s working, you don’t need to replace it; the review simply gives you a reason to keep it, tune it or plan an upgrade before something fails.
KNOW THE LIMITS
When to get expert help
Get product-specific or professional help for anything involving floor loading, household electrics, unsafe plumbing, damaged glass, unexplained livestock deaths, or chemistry you can’t measure reliably. Use a qualified electrician for fixed wiring, and always follow the current safety instructions for your tank, equipment and additives. Never work around damaged cables, swollen batteries, leaking seals or equipment that keeps tripping the electrics.
Your livestock’s needs can also outweigh general equipment advice. Check adult size, behaviour, disease risk and each species’ light, flow and feeding needs with reputable care guides before you buy. This guide explains the equipment decision; it can’t guarantee the result for every animal or tank.
COMMON QUESTIONS
Frequently asked questions
Are calcium reactors cheaper to run?
They can be when demand is consistently high, but compare the full cost of equipment, CO₂, media and upkeep.
Do calcium reactors lower pH?
Their CO₂-rich output can pull pH down. How much depends on aeration, tuning and your setup.
Can a controller make either method safe?
It can monitor and stop some failures, but you still need your own tests, calibration and physical safeguards.