How to use the pH Monitor
Log your pH from CO₂-on to lights-on and watch your dissolved CO₂ build in real time. This guide walks through setup, what each column means, how to read the status badges, and what to expect from a typical session.
Planted aquariums need dissolved CO₂ to drive plant growth, but the margin between effective and dangerous is narrow. Too little and plants stagnate; too much and fish show stress, gulp at the surface, or worse. The problem is that you cannot measure dissolved CO₂ directly with a consumer test kit. What you can measure is pH — and pH tracks CO₂ closely enough that logging it at regular intervals gives you a reliable picture of what's happening in the water column.
The AquaCalc pH Monitor turns that log into a live dashboard. Every reading you enter triggers a CO₂ estimate, a progress bar, and a colour-coded status badge. By the time your lights come on you have a complete record of the session — one you can share, review, and compare to future sessions to dial in your needle valve.
Step 1 — Set up your tank profile
Before you log a single reading, fill in the Tank inputs panel on the right side of the dashboard. These values are saved to your profile automatically and carried into every calculation:
- KH (dKH) — your carbonate hardness. The KH/pH formula uses this to convert pH into a dissolved CO₂ figure. If you use aquasoil and don't know your KH reliably, use pH drop mode instead (see below).
- Tank volume — used by the dosing and water change calculators; not directly needed for CO₂ logging but worth filling in now.
- Target CO₂ (ppm) — typically 20–30 ppm for a planted tank. This is what the progress bar and status badges count toward.
- Danger limit (ppm) — the CO₂ level at which the tool fires a red DANGER alert. 40 ppm is the commonly cited threshold for fish stress; lower it if your stock is sensitive.
- Temperature — affects the CO₂ equilibrium. The KH/pH formula is temperature-sensitive; the same pH and KH at 24°C versus 28°C represent slightly different actual CO₂ levels. AquaCalc applies a temperature correction to the formula (it shifts the carbonic-acid pK₁ with temperature), so your CO₂ figure stays accurate across the normal aquarium range of roughly 15–35°C — most calculators skip this.
Step 2 — Log your overnight pH first
The single most important reading of any session is the one you take before CO₂ turns on — ideally first thing in the morning before lights come on, after the CO₂ has been off all night. This is your baseline. The tool labels the first row CO₂ on and uses that pH as the reference point for everything that follows.
"Your overnight pH is the anchor for the whole session. A stable overnight pH — the same reading morning after morning — is a sign that your tank's buffering capacity is well understood."
If you use the pH drop method (for aquasoil tanks where KH is unreliable), this overnight reading is even more critical: the entire CO₂ estimate is derived from how far pH has dropped from that baseline.
Step 3 — Generate the log table
At the top of the pH reading log, set three things:
- CO₂ on at — the time you switch your CO₂ on (or your solenoid opens). The table generates time slots from this point forward.
- Interval — 15, 30, or 60 minutes between readings. 30 minutes is the most common choice; it gives you enough resolution to see the curve without requiring you to be at the tank constantly.
- Slots — how many rows to generate. 8 slots at 30-minute intervals covers 4 hours, which is enough for a CO₂-on-to-lights-on window in most setups.
Hit Generate. The table fills with empty rows, each labelled with a time. The first row defaults to CO₂ on; you can change any row's event type using the dropdown.
What to enter and when
At each interval, check your pH meter and type the reading into the row for that time. Press Tab to move to the next row — the cursor jumps automatically. The row calculates immediately: you'll see the estimated CO₂, the pH drop from baseline, the progress bar, and the status badge update as you type.
You don't need to enter every row. If you miss a slot, leave it blank — the tool skips blank rows in calculations and the next reading you enter picks up from where the last one left off.
Marking events
Use the Event dropdown on any row to mark what happened at that point in the session. Options:
- CO₂ on — the first row; CO₂ entering the water.
- pH reading — a standard interval reading with nothing notable happening.
- Lights on — mark the row where your lights come on. Plants begin consuming CO₂ immediately; you'll often see the pH drop slow or reverse from this point.
- CO₂ off — mark when your solenoid closes. Useful if your CO₂ turns off before lights-out and you want to track the recovery curve.
- Lights off — end of the photoperiod.
The CO₂ curve — what to expect
A typical planted tank CO₂ session follows a predictable curve, and understanding its shape helps you interpret the readings rather than just recording numbers. One important note before you start: a gradual rise from 20 ppm at lights-on to 30 ppm mid-session is normal and healthy — it is not instability. Our CO₂ stability guide explains what the science says about what "stable" actually means and what genuinely causes algae problems.
When CO₂ first turns on, dissolved levels in the water are near zero — all the CO₂ from the previous day has offgassed overnight. The gas dissolves quickly into the water column and pH drops at its fastest rate in the first 30–60 minutes. This is the Building phase.
As dissolved CO₂ climbs, the rate of dissolution slows. The water becomes increasingly saturated and the equilibrium between CO₂ entering via the diffuser and CO₂ escaping at the surface begins to balance out. The pH drop that looked dramatic in the first hour flattens to a gentle slope. This is the Approaching phase — the curve is still heading in the right direction, just more slowly.
"CO₂ does not build linearly. The first hour looks dramatic; the second hour looks flat. If you only see fast drops you're still in early build-up. If the drops have slowed to a crawl, you're close to your equilibrium point."
Once you hit target, CO₂ levels stabilise. The needle valve is holding a steady flow and the water is at saturation for that flow rate. This is At target — the badge turns green and holds there as long as your CO₂ supply and agitation are consistent.
When lights come on, plants begin photosynthesising and consuming CO₂ directly. You'll typically see pH stop falling or even tick up slightly in the first few readings after lights-on. This is normal — the plants are helping maintain CO₂ levels by using what's dissolved. In a heavily planted tank with strong light, the consumption can be significant enough to require a higher CO₂ flow rate to maintain target levels through the photoperiod.
Reading the status badges
Every row shows a status badge that tells you where you are relative to your target. For the KH formula mode:
- Building — CO₂ is rising but still well below target. Normal for the first 30–60 minutes.
- Approaching — past 70% of target. Getting close; start watching more carefully.
- ✓ At target — within 5% of your target CO₂. This is where you want to be through the photoperiod.
- Over target — CO₂ has exceeded your target ppm. Consider easing the needle valve slightly.
- DANGER >40ppm — CO₂ has exceeded the danger threshold. Fish may be stressed. Turn down CO₂ and increase surface agitation immediately.
In pH drop mode, the danger alert works the same way, but the build-up progression is more granular and uses its own labels: Starting → Building → Climbing → Nearly there → ✓ At target → Over target → DANGER. (Over target here means the drop has gone about 15% past your target drop, giving a little headroom before it flags.)
KH formula vs pH drop — which to use
The tool offers two calculation modes, selectable in the Tank inputs panel:
KH / pH formula
Uses the standard carbonate chemistry equation to convert your KH and measured pH into a dissolved CO₂ figure in ppm. This is the most accurate method when your KH is stable and reliably measured. It works well with hard tap water and gives you an absolute CO₂ reading at every time slot. For the science behind the formula itself, see the KH, CO₂ and pH guide.
The main limitation: aquasoil and other CEC substrates actively absorb and release ions, making KH readings unstable and unreliable. If your KH changes week to week despite no water changes, the formula will give misleading results. The substrate CEC article explains exactly why this happens.
pH drop method
Ignores KH entirely and measures CO₂ build-up as a function of how far pH has dropped from your overnight baseline. You set a target pH drop (e.g. 1.0 unit) that corresponds to your target CO₂ level, and the tool tracks progress toward that drop. This method is standard practice for aquasoil tanks where KH cannot be trusted.
One honest caveat: the absolute ppm figure in this mode is an estimate (shown with a “≈”). It assumes your degassed, overnight water sits at roughly 3 ppm CO₂, so each 1.0 unit of pH drop represents about a ten-fold rise. Treat the trend and the progress toward your target drop as the reliable signal, rather than the exact ppm number.
pH reading alerts
Once you have generated a log table, the 🔔 Alerts off button appears in the log controls (desktop only). Click it to switch alerts on — the button turns green and shows 🔔 Alerts on. From that point, the tool fires a browser notification at each scheduled reading time, so you don't need to watch the clock.
A countdown display appears showing exactly how long until the next unfilled reading slot is due. When the time arrives and you haven't yet entered a reading, a banner fires at the top of the log. Alerts skip any slot where you've already entered a reading.
You will need to grant notification permission the first time you enable alerts — your browser will prompt you. Alerts only fire while the tab is open; they are not push notifications.
Analyse curve — CO₂ profile report
Once you have readings logged, a ✶ Analyse curve button appears alongside the Share button. Clicking it generates a full analysis of your CO₂ session — grounded in the same science as the CO₂ stability guide.
The report opens as a modal and shows:
- The CO₂ curve — a snapshot of your session chart trimmed to the readings you have entered, so you can see the shape of the session alongside the findings.
- Overall verdict — one of five ratings: Excellent, Good, Needs work, Poor, or Critical. Each comes with a plain-English summary of what that means for your plants.
- Session stats — peak CO₂, CO₂ at lights-on (if you have marked that event), session average, and the percentage of readings at or above the 20 ppm effective floor for plant photosynthesis.
- Findings — individual observations about your curve, each colour-coded by severity. Green findings are things going right; amber findings are issues worth addressing; red findings are problems that need immediate attention. Each finding that requires action includes a specific suggested fix.
The analysis checks for: mid-session crashes (sharp CO₂ drops between readings), danger levels exceeded, CO₂ critically or chronically low, CO₂ still low at lights-on, target not reached, an oscillating or unstable profile, CO₂ declining after peak, and whether the session spent meaningful time in the effective range.
Your readings save automatically
Every pH value you type is saved to your browser profile as you go. If you accidentally close the tab, reload the page, or switch to your phone, your readings are still there. Nothing is lost between sessions as long as you're on the same browser and device.
Profiles are identified by an anonymous ID stored in your browser — there's no account to create, no email address required, and no data collected that identifies you.
Sharing your session
Once you have readings logged, the ↗ Share session button appears in the log controls. Click it and AquaCalc generates a unique link to a branded page showing your full session: pH and CO₂ charts, a colour-coded readings table with all your event markers, and your tank parameters. The link is valid for 90 days.
Paste it into a forum thread to get feedback on your CO₂ curve, share it with a friend who's setting up their first pressurised system, or just keep it as a record of a dialled-in session.
Backing up and restoring your profile
Your profile is stored on AquaCalc's server and tied to your browser. If you want a local backup — or you're switching to a new device or browser — use the ↓ Export button in the profile bar at the top of the dashboard. This downloads your entire profile as a .json file: all tank settings, RO mixer parameters, dosing preferences and any saved pH readings.
To restore it, click ↑ Import and select the file. AquaCalc loads the profile immediately and saves it to the server. If a profile with that name already exists you'll be asked whether to overwrite it or keep both.