Which Fish Can Handle Plain Tap Water, and Which Can’t — pH and Hardness

Written 2026-10-22

Some fish do fine straight from the tap; others need their water softened and made more acidic first — and the difference comes down to pH and hardness (GH). Running the Water Kiwoogi simulator on a 60cm tank filled with plain tap water (KH 4.0, GH 7.0), the pH settles around 7.5-7.7. Guppies, which like neutral-to-hard water (ideal pH 7.4), come through 90 days with all 10 fish healthy. Cardinal tetras, which want soft, slightly acidic water (ideal pH 6.2, max 7.0), spend the whole 90 days with the tank’s pH sitting above their comfort range, and six of the ten die off one by one, leaving only four. Mixing in 50% RO water to bring the starting pH down from 7.4 toward 6.7 changes that — only one fish is lost early on, and nine survive.

Being outside the pH range kills slowly, not instantly

In this simulator, every species has a preferred pH range ([min, ideal, max]), and a tank whose pH sits inside that range causes zero stress. Stress only builds for the part that’s outside the range, and unlike ammonia or nitrite, it doesn’t show up in a day or two — it wears a fish down over weeks or months. In the plain-tap cardinal tetra tank, the first death on day 6 was mostly nitrite (71%) with some pH (29%), but the later deaths on days 21, 50, 78 and 88 are each logged as “cause: pH 100%.” Getting a tank through the initial cycling period doesn’t mean a mismatched species is safe — if the pH stays wrong, fish keep dying long after that.

Which fish fit plain tap water, and which don’t

Going by this simulator’s catalog, livebearers such as guppies, platies, mollies and swordtails, along with zebra danios, pearl danios, white clouds and galaxy rasboras, sit comfortably in ordinary tap water (KH 3-5, pH around 7.3-7.8) — their ideal pH is 7.0 or higher, or their range stretches up into the 8s. Species such as cardinal tetras, green neon tetras, rummynose tetras, German blue rams and Apistogramma cacatuoides are the opposite — their ideal pH is in the 6s with a max around 7.0, so plain tap water leaves them outside their range for months at a time.

pH matters far more than hardness (GH)

Tap water’s GH of 7.0 is a bit below the range guppies prefer (8-25), yet all ten guppies came through fine — in this simulator, being outside the GH range carries only about a tenth of the stress weight that being outside the pH range does. So a GH reading that’s slightly off from a species’ catalog range usually isn’t worth worrying about; checking whether the pH sits inside the range matters much more.

What to do if your tap water doesn’t fit

If you want to keep a species that prefers pH in the 6s but your tap water sits around 7.5, mixing in roughly half RO (reverse osmosis) water helped in this simulator — the 50% RO setup above starts at pH 7.37 and drifts down to 6.74 by day 90. If that still isn’t low enough, you can raise the ratio further, but pushing it too far can strip out so much KH that the buffer disappears and pH crashes straight down to 5.0 — so raise the ratio gradually and keep an eye on KH rather than jumping straight to a high one. When in doubt, check with your local aquarium shop or an experienced keeper.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.