A useful water test log keeps the date, the reading, its unit and some context together. Only record the tests you actually did, leave anything you didn’t test blank, and keep zero as a real result. Consistent notes make it much easier to read a trend without mixing up different methods or units.
Open the free water test log →
What goes in each record
The Companion has fields for alkalinity, calcium, magnesium, nitrate, phosphate, salinity and temperature. You can enter one reading or several for the same date. Partial tests are fine, so you never need to make up numbers to fill the form.
Use the note for the kit or meter, the method, timing and anything unusual about the test. If you tested after a water change, say so. The log is a record of what you measured; it doesn’t certify your test or judge your tank’s health automatically.
| Reading | Unit in the log | Check before entering |
|---|---|---|
| Alkalinity | dKH | Only convert from another unit using a proper method |
| Calcium and magnesium | mg/L | Use the result and scale from your test |
| Nitrate | mg/L NO₃ | Check whether your test reports nitrate or nitrogen |
| Phosphate | mg/L PO₄ | Check whether your test reports phosphate or phosphorus |
| Salinity | ppt | Don’t enter a specific gravity number here |
| Temperature | °C | Convert from Fahrenheit first |
Keep blanks and zeros apart
A blank nitrate field means you didn’t enter a nitrate reading. A nitrate value of zero means you recorded zero. Those say different things and give different charts, so the tool leaves blanks out rather than plotting them as zero.
A reading below what your test can detect isn’t proof there’s none present, either. Put that in the note and follow your test’s instructions. The log can’t work out a detection limit from a number alone.
Read the chart with its scale in mind
Pick one reading and a 30-day, 90-day or all-records view. The chart plots your readings by date, with the latest value and number of records shown above it. A table underneath gives the numbers if you’d rather compare them in writing.
The vertical scale fits itself to your readings, so a small change can look big. Check the axis and the actual differences before reading too much into the shape. The lines join your readings; they don’t show what happened in between. Several tests on the same day stay as separate records.
For example, made-up alkalinity readings of 8.0, 8.2, 7.9 and 8.1 dKH span 0.3 dKH. The chart helps you notice that spread, but can’t tell you the cause or whether you need to act. The sample tank is there to show how the screen works.
Fix a mistake without losing a real result
Use the edit button to correct a date, a unit mix-up or a typo. Editing replaces that record’s values. If you repeat a test for real, add a new record instead, so the earlier reading stays in your history rather than vanishing.
Switching kits, calibrating a meter or changing how you test can all affect whether results compare. Note those changes. A chart alone can’t tell a real change in your tank from a change in how you measured.
Keep a copy you control
The free version only saves when you choose “Save Companion on this device”. That copy belongs to this browser and doesn’t sync to another phone or computer. Download a JSON copy before clearing site data or switching devices.
A full export includes your water tests along with your tank, tasks, completion history and spending. Keep personal details out of your notes, especially on a shared device. Use your records to help you think things through, not as automatic dosing instructions.
Common questions
Do I have to enter every reading?
No. Enter the tests you did. A record needs at least one reading; the other fields can stay blank.
Can I enter specific gravity as salinity?
No. The salinity field uses ppt, so don’t paste a specific gravity reading into it. Get the right value from your instrument using a suitable method.
Does the chart tell me what to dose?
No. It shows your readings and their dates. It doesn’t diagnose livestock, recommend dosing or label results as safe.
Sources and limits
Last checked 12 October 2026. Our worked examples use made-up inputs to show the maths. They aren’t measured costs or care targets for your tank.
Keep going
Open the free water test log · Free Reef Companion · All calculators