Medaka Water Parameters

The one rule over all of it: stability beats perfection

Most beginner water problems are not caused by a bad number. They are caused by a number moving fast: a big water change with mismatched water, a pH crash, a chemical added to fix something that was not broken. Medaka handle a wide range of steady conditions and handle sudden change poorly, which is the same thing my temperature guide says about heat and cold. When in doubt, change less, slower.

pH: wide comfort, gentle hands

Medaka are comfortable roughly from pH 6.5 to 8.5, with neutral to slightly alkaline the common sweet spot. Nearly every municipal tap in America lands inside that band, which means your tap water pH is almost certainly fine as it is. The classic beginner mistake is buying pH-up or pH-down chemicals and riding the number like a thermostat; every dose is a sudden change, and sudden change is the actual enemy. Test it once out of curiosity, then leave it alone.

KH: the bodyguard nobody tests

Carbonate hardness (KH) is the buffer that holds pH steady. Plenty of KH, stable pH. Low KH, and pH can swing hard, especially in heavily planted water, because photosynthesis itself moves the chemistry over the course of a day. If your pH ever seems to wander or crash, KH is the first thing to test, not the last. Raising it is gentle and cheap; a small bag of crushed coral in the water is the classic slow fix, and it only works while the water is on the acidic side. General hardness (GH), the mineral content, matters less: medaka adapt to soft and hard water alike, and ordinary tap water is fine.

Ammonia and nitrite: the only true emergencies

These two are different. Ammonia (from fish waste and rotting food) and nitrite (the halfway product of breaking ammonia down) are both toxic, and the only acceptable reading for an established setup is zero. Any detectable amount is a real problem: stop feeding or feed very lightly, do a partial water change with dechlorinated water, and find the cause, which is nearly always overfeeding, overstocking, or a setup that was too new for fish.

The cycle, in one paragraph

"Cycling" just means growing the bacteria that eat ammonia and nitrite. They live on every wet surface, they arrive on their own, and they multiply to match the waste available. That is the whole reason for the two-week wait in my mini pond guide: you are not waiting on the water, you are waiting on an invisible workforce to move in. Plants shortcut the process beautifully because they eat ammonia directly, which is why a heavily planted container is so much more forgiving than a bare one.

Nitrate: the slow accumulator

The end of the chain, far less toxic, and the reason aquariums need water changes. Keep it modest, under roughly 40 ppm is a common hobby ceiling, and lower is better. In a planted outdoor container this number often stays low on its own, because the plants are eating it as fertilizer. Rising nitrate in a pond usually just means the floaters need thinning less often than you thought, or the feeding hand got generous.

Chlorine and chloramine: the silent fish-killer in the hose

Tap water is disinfected on purpose, and what kills germs also burns gills. Chlorine gasses off if water stands a day or two; chloramine, which many cities now use, does not. A cheap water conditioner neutralizes both within minutes (with chloramine it also frees a little ammonia that most conditioners bind, so a fresh fill can briefly read a trace on a test) and is the single most non-negotiable bottle in this hobby. Every fill and every top-off, condition first.

What I actually do

My pots get conditioned top-offs, plants doing the daily work, modest feeding, and a test kit that comes out when a new pot is maturing, not on a schedule. In an established pot it mostly stays in the drawer, because a planted, lightly stocked container is a system that holds itself steady, which is the entire philosophy of how I raise fish here. Buy the test kit, learn what the numbers mean, and then earn the right to mostly ignore them.

Rainwater, the top-off the fish prefer

Plenty of my top-offs are simply rain falling into the pots, and both the fish and the plants respond to it. There is real chemistry behind that. Rainwater arrives with no chlorine or chloramine at all, so nothing needs neutralizing. It is naturally soft and very low in dissolved minerals, which is close to the paddy water medaka evolved in, and it even carries a whisper of dissolved nitrogen that plants can use. It is what outdoor systems have run on forever. One caution: because rainwater has almost no KH, it does not buffer pH on its own, so use it for top-offs into an established pot rather than filling a container with nothing but rainwater. And collect it clean; the first runoff from a roof can carry whatever was sitting on the shingles.

pH	Roughly 6.5 to 8.5, steady; do not chase it with chemicals
KH	The pH buffer; test it first when pH misbehaves
GH	Flexible; ordinary tap water is fine
Ammonia	Zero, always; any reading means act, and above about 0.25 means act today
Nitrite	Zero, always; any reading means act, and above about 0.25 means act today
Nitrate	Keep under roughly 40 ppm; plants eat it
Chlorine	Condition every fill and top-off of fish water, no exceptions

Common questions

What pH do medaka need?

Medaka are comfortable across a wide band, roughly pH 6.5 to 8.5, with neutral to slightly alkaline being the common sweet spot. Nearly all US tap water lands in that range already. A steady pH matters far more than a perfect one; chasing a specific number with chemicals causes more harm than the number itself ever would.

Do I need to test my pond water?

A basic test kit is worth owning for three moments: before the first fish go in, when something seems wrong, and occasionally in a new setup's first month. A mature, planted, lightly stocked container that looks healthy rarely needs routine testing. Test with purpose, not as a ritual.

Why did my pH crash overnight?

Usually low KH (carbonate hardness). KH is the buffer that holds pH steady; when it runs low, pH can swing hard, especially in heavily planted water where photosynthesis moves the chemistry daily. If your pH is unstable, test KH before blaming anything else, and raise it gently, a small amount of crushed coral is a common slow fix when the water runs acidic.