An aquarium log book should record every water test, every water change and dose, every animal added or lost, equipment changes, and anything unusual you notice, each with a date. Its job is to answer “what changed?” when something goes wrong, which you can’t reliably do from memory three weeks later.
You don’t need to write much. A short, consistent entry every time you work on the tank beats a detailed diary you abandon after a month.
What should you write in an aquarium log book? #
| Entry | What to write | When |
|---|---|---|
| Water tests | Date, time, every reading, the kit used | Every test |
| Water changes | Amount (percent or volume), water source, salt brand if saltwater | Every change |
| Doses and additives | Product, amount, reason | Every dose, or when you change a daily dose |
| Livestock added | Species, number, source, date, cost, quarantine notes | Every addition |
| Livestock lost | Species, date, what you saw beforehand | Every loss |
| Medications | Product, dose, start and end dates | Every treatment |
| Feeding changes | New food, new amount, new schedule | When it changes |
| Equipment | Installs, cleaning, replacements, failures | When it happens |
| Observations | Behavior, color, algae, cloudy water, smells | Whenever something looks different |
Tests and water changes are the backbone. The observations column is what turns a list of numbers into a history you can use.
Why record the kit you used? #
Different kits read differently. If you switch from test strips to a liquid kit, your nitrate may appear to halve overnight. Writing the kit next to the reading keeps you from reacting to a change that isn’t real. Our guide on how to test aquarium water covers getting consistent readings.
Why record the time? #
pH and, in reef tanks, alkalinity shift through the day with the lights. A pH of 7.9 at 7 a.m. and 8.2 at 7 p.m. can both be normal. Testing at the same time each day, and noting it, makes readings comparable.
What does a good log entry look like? #
Short and specific. Here are three sample entries for an imaginary 40-gallon freshwater tank:
2026-09-06 19:30
Test (API liquid): ammonia 0, nitrite 0, nitrate 20, pH 7.4, temp 78F
Water change: 25%, tap water + conditioner
Notes: nitrate up from 10 last week. Fed extra while away? Rinsed sponge in tank water.
2026-09-09 08:15
Added: 6 harlequin rasboras (local store), $4 each. Drip acclimated 45 min.
Notes: one rasbora hiding, others schooling.
2026-09-13 19:45
Test (API liquid): ammonia 0.25, nitrite 0, nitrate 20, pH 7.4
Notes: ammonia first time in months. New fish + filter rinse on the 6th? Retest tomorrow. Cut feeding by half.The third entry shows why logs matter. Reading back a week points straight at two likely causes: more fish, and a filter rinse that may have knocked back the bacteria.
How often should you make entries? #
Match the tank’s stage:
- New or cycling tanks: test and log every day or two until ammonia and nitrite read zero.
- Established freshwater tanks: log tests weekly or every other week, plus every water change and change to the tank.
- Reef tanks: log alkalinity several times a week while you’re setting doses, calcium and magnesium weekly, and nutrients weekly.
- Any tank: log on the day something happens, whether that’s a new fish, a death, a heater swap or a strange smell.
Your maintenance tasks follow a schedule of their own. Our fish tank maintenance schedule lays out what to do daily, weekly and monthly, and the log is where you tick those tasks off.
Paper log book, app or spreadsheet? #
| Paper log book | Logging app | Spreadsheet | |
|---|---|---|---|
| Cost | A notebook | Free tiers; some paid | Free |
| Entry at the tank | Easy if it stays dry | Easy | Awkward on a phone |
| Reminders | None | Push notifications | None |
| Charts | Draw them yourself | Automatic | Build once, then automatic |
| Searching old entries | Flip pages | Scroll or filter | Filter and sort |
| Risk of loss | Water damage, lost notebook | Phone loss if data is local | Low if backed up |
| Custom fields | Anything | A fixed list | Anything |
Paper works well if you keep it by the tank and actually write in it. Its weakness is spotting trends, because a slow climb in nitrate is hard to see across ten pages. A spreadsheet solves that and handles any field, and our spreadsheet vs app comparison includes a layout to copy. Apps win when you want reminders and don’t want to build anything.
Fish Tank Tracker is laid out like the table at the top of this page. Each water test holds up to ten readings plus a notes field for the kit and observations. It has a water change log (percent or volume), a livestock journal with species, photos, purchase dates, cost and health status, and an equipment list with install dates and maintenance history. Recurring tasks send a reminder when they come due. Everything stays on your phone with no account. There’s no separate dosing or medication log, so those go in the test notes. The free tier also keeps a month of parameter history, so if you want a multi-year record, either buy the one-time Pro upgrade or keep a spreadsheet alongside.
How do you use a log book when something goes wrong? #
When fish start dying or a reading jumps, work through the log before reaching for a treatment:
- Write down today’s problem with a full set of readings.
- Read back two to three weeks. Most problems have a cause in that window.
- List every change in that period: new animals, food, doses, medications, equipment work, water source, a skipped water change.
- Compare readings to the tank’s normal range, not just a generic chart. Your log shows what normal looks like for this tank.
- Change one thing at a time and keep logging, so you can tell what worked.
That process only works if the log has entries from when things were fine. The dull entries are the baseline that makes the alarming one mean something.
Frequently asked questions #
Should I record feeding in my aquarium log? #
Record changes, not every meal. Note when you switch foods, change amounts or go away and use an automatic feeder. Overfeeding causes many ammonia and nitrate problems, and a feeding note is often the clue.
Do I need to log tests when everything is normal? #
Yes. Normal readings set your tank’s baseline, and without them you can’t tell whether today’s nitrate of 30 ppm is a sudden jump or where the tank always sits.
What’s the difference between a log book and a maintenance schedule? #
A maintenance schedule says what should happen and when, like a water change every week or a filter rinse every month. The log book records what did happen. You need both, and many keepers tick off scheduled tasks in the log.
How long should I keep an aquarium log? #
For the life of the tank. Old records show seasonal patterns, how long equipment lasts and how the tank responded to past problems, which is useful years later.