pH generates more anxiety per unit of actual risk than anything else in betta keeping. The short version: read it once, write it down, and get on with your life.
The range
Anything stable between roughly 6.5 and 8.0.
That is wide on purpose. Wild bettas live in soft acidic water, and the fish in the shop is not a wild betta. It has been bred for many generations in ordinary municipal tap water at whatever pH the local supply runs at, and it is adapted to that far more than to a rice paddy.
If your tap water is 7.8, keep your betta at 7.8.
Why change is the actual danger
A fish regulates its internal chemistry against the water around it. Holding a steady value, even one at the edge of the comfortable range, is something it does without difficulty.
Moving that value repeatedly is not. Each shift asks for an adjustment, and a fish adjusting on a loop is spending on that what it should be spending on staying healthy.
This is why the useful measurement is not today’s reading but whether the reading is the same as last month’s.
The trap
Someone tests, sees 8.0, reads that bettas prefer acidic water, and buys a pH-down product.
The product lowers the value. The carbonate hardness in the water buffers it back up over a day or two. They redose. The fish now lives on a see-saw that did not exist before anyone tested anything.
The original 8.0 was harmless. The correction was not.
When pH genuinely needs attention
Two situations, and neither is about the number being wrong.
It will not stay put. Moving noticeably week to week points at low carbonate hardness, meaning nothing is anchoring it. Adding a small buffer, such as crushed coral in the filter, gives it something to hold on to. That treats the cause rather than the symptom.
It changed suddenly. A sharp move usually means something else happened: a large water change with different water, a dead animal decomposing, a new piece of decor leaching, or a filter that stopped. Find that cause.
Gentle influences, if you want one
Driftwood and Indian almond leaves release tannins that lower pH slightly and tint the water amber. The effect is gradual and self-limiting, which makes it safer than a bottled adjuster, and many keepers like the look and the more natural feel.
Treat it as a nudge rather than a control. If your water is strongly buffered, almost nothing will happen, and that is fine.
How to test it
Liquid kit, in daylight, against something white. Test once when the tank is set up, note the number, and check again only if something changes.
Do not test daily. Small natural variation across a day is normal, and watching it closely is how people talk themselves into fixing a problem they do not have.
Where to go next
The water parameters guide covers every reading and what each one does to the fish. The tap water article covers your source water, and the testing walkthrough covers getting a reading you can trust.