Find out which direction the swing runs
A pH that reads one value in the morning and another by evening is telling you something specific, so measure before you fix. Test the tank at the same times for a few days, and also test a cup of your tap water left standing overnight — tap supplies often leave the treatment works at one pH and settle to another once dissolved gases escape, which means some swings are built into every water change. Swings downward between changes point to acids building up in the tank; swings upward after changes point at something alkaline in the decor or the supply.
The usual culprit in small tanks: low buffering
Carbonate hardness, the KH on a test kit, is the water's buffer — it soaks up acids as waste breaks down and as plants and bacteria work. Small tanks often run on soft water with little KH, so the acids from feeding, fish waste, and bogwood steadily exhaust the buffer and the pH slides, sometimes quickly once the buffer is gone. The smaller the volume, the faster it happens, which is why a nano tank can swing in days where a large tank drifts over weeks. Raising and holding a modest KH is what steadies the pH; chasing the pH number directly treats the reading, not the cause.
Stabilising the routine, not just the reading
Change less water more often: small, frequent changes with water matched for temperature and chemistry nudge the tank gently instead of replacing a large share of its water with different water in one shock. Prepare change water the same way every time — if you add a buffering product or blend in mineral-rich water, do it to the bucket, measure it there, and only then add it. Inside the tank, remove the drivers: siphon out decaying food, rinse filter media on schedule so trapped waste is not acidifying the water, and check that a new rock, gravel, or ornament is not quietly dissolving into the water column.
What not to do
Avoid liquid pH-up and pH-down adjusters as a standing routine. They force the number to a target for a few hours, the tank's chemistry pushes back, and the fish live through the bounce both ways — a stable pH slightly off a species' textbook ideal is far kinder than a perfect number that will not sit still. Sudden swings are also a reason not to do a huge emergency change with unmatched water: you can turn a drift into a shock in one bucket.
When the swing is a symptom, not the problem
A pH that crashes despite steady buffering, or fish gasping and flashing while readings wander, can mean the filter is failing, the tank is overstocked for its volume, or something has died out of sight and is fouling the water. Test ammonia and nitrite alongside pH, count your livestock, and if fish are in visible distress while the numbers keep moving, take a water sample to an aquatic shop or aquatic vet for a full test rather than adjusting blind. Keeping a running log of every test result in your fish's Pet Passport on KinsLe is what turns a week of confusing numbers into a visible trend you can act on.