Most betta problems that look like illness are water problems. The fish is the symptom. The water is the cause.
This guide covers what to test, what the numbers mean, and what to actually do at each reading. If you only take one thing from it: ammonia and nitrite must read zero, and nothing else on this page matters as much.
The numbers, in one table
| Parameter | Target | Acts on your fish |
|---|---|---|
| Ammonia | 0 ppm | Burns gills and fins. Lethal well before it is visible |
| Nitrite | 0 ppm | Stops blood carrying oxygen |
| Nitrate | Under ~20 ppm | Slow stressor. Removed by water changes |
| Temperature | 25 to 28 °C | Below range suppresses the immune system |
| pH | Stable, ~6.5 to 8.0 | Sudden change is the danger, not the value |
| GH | 3 to 15 dGH | Very tolerant. Rarely worth adjusting |
| KH | 3 to 8 dKH | Low KH lets pH swing |
The first two are pass or fail. The rest are ranges, and a stable value slightly outside a range beats a perfect value you keep chasing.
Ammonia: the one that kills
Your fish excretes ammonia constantly, and uneaten food produces more. In a cycled tank, bacteria convert it within hours. In an uncycled tank it accumulates.
Ammonia burns gill tissue and fin edges. By the time you can see the damage the exposure has been going on for days. There is no safe visible level: 0.25 ppm is already doing harm.
If you read anything above zero:
- Change 50 percent of the water now, dechlorinated and temperature matched.
- Stop feeding for a day or two, since less food means less ammonia.
- Test again in 24 hours, and repeat the change until it reads zero.
- Then work out why. Usually the tank was never cycled, or the filter media was rinsed in tap water and the colony died with it.
Water changes are first aid, not a cure. The cure is a working cycle.
Nitrite: the one people forget
Nitrite is the second stage of the cycle and is also toxic. It interferes with the blood’s ability to carry oxygen, so a fish in nitrite may gasp at the surface even though the water is well oxygenated.
Treat a nitrite reading exactly like an ammonia reading. Same response, same urgency.
Nitrate: the one you manage
Nitrate is the end of the cycle and is far less harmful. It builds up steadily and is removed by water changes, which is most of the reason water changes exist.
Under about 20 ppm is comfortable. Rising nitrate usually means you are changing water too rarely, feeding too heavily, or both. Live plants use some of it, but plants are not a substitute for maintenance.
Temperature: stability over precision
Bettas are tropical. Target 25 to 28 °C.
The number matters less than the steadiness. A fish held at 25 °C all year is better off than one that reaches 27 °C each afternoon and drops to 21 °C overnight. Repeated chilling suppresses the immune system, and a great deal of what gets treated as mystery illness is a fish that has been cold too often.
Practical notes:
- Use an adjustable heater and a separate thermometer. Heater dials are frequently wrong.
- Check the thermometer, not the dial, when you check the tank.
- Heaters fail, sometimes stuck on. A thermometer is what tells you.
- Do not chase a degree. Chase consistency.
pH: leave it alone
Bettas are far more tolerant of pH than the internet suggests. Wild fish come from soft acidic water, but the fish you bought was bred for generations in ordinary tap water.
Anything stable in roughly the 6.5 to 8.0 range is fine. What harms fish is change, and pH-adjusting products cause exactly that: they move the value, the water’s buffering pulls it back, and the fish rides the swing.
If your tap water is 7.8, keep your betta at 7.8.
GH and KH: the ones nobody explains
GH is dissolved calcium and magnesium. KH is the water’s resistance to pH change.
GH matters little for bettas across a wide range. KH matters indirectly: when it is very low, pH loses its anchor and can move quickly. If your pH will not stay put, low KH is usually why, and the fix is a small amount of buffering rather than a pH product.
Most keepers can test these once, note the values, and never think about them again.
How to test, and how often
Buy a liquid kit. Strips struggle exactly where accuracy matters, at the low end of ammonia and nitrite, and the difference between 0 and 0.25 ppm is the difference between fine and harmful.
Read the results in daylight against something white. Tank lighting shifts the colours enough to mislead.
Frequency:
| Situation | How often |
|---|---|
| Cycling a new tank | Daily |
| First month with a new fish | Every two or three days |
| Established and stable | Weekly |
| Anything looks wrong | Immediately, before anything else |
That last row is the habit worth building. When a betta looks off, test the water before you reach for a medication. Most of the time the water explains it, and medicating a water-quality problem makes it worse.
Reading your results
| What you see | What it means | What to do |
|---|---|---|
| Ammonia or nitrite above 0 | Not cycled, or the colony is damaged | 50% water change now, repeat daily |
| Nitrate climbing past 20 | Not enough water changes, or overfeeding | Larger or more frequent changes |
| Everything 0, including nitrate | Often an uncycled new tank, not a clean one | Keep testing; a cycled tank produces nitrate |
| pH moved a lot in a week | Low KH, or a product is fighting the water | Stop adjusting, check KH |
| Temperature swinging | Heater undersized, failing, or absent | Replace, and verify with a thermometer |
The third row surprises people. Zero across the board on a young tank usually means nothing is happening yet, not that the water is pristine.
Where to go next
Cycling is the process that keeps ammonia and nitrite at zero on their own, and it is the single most useful thing to get right before a fish goes in the tank.