What Your Water Test Results Mean
You tested the water, and now you are staring at a strip or a row of little vials, holding numbers with no instructions. This guide is the instructions. It reads the way I read a test: worst number first, calm plan for each, and no chemicals unless the water has genuinely earned them. My water parameters guide explains the chemistry behind all of this; this page is for the moment you have results in hand.
Read your results worst-first
Not all bad numbers are equally bad. Ammonia and nitrite are the only true emergencies, so check those two first, and if either is above zero, deal with it before you give the others a thought. Nitrate, pH, hardness, and temperature are slower stories: they matter, but they hurt fish over weeks, not hours. The sections below run in that order, most urgent to least.
Ammonia: the only good number is zero
Ammonia is fish waste and rotting leftovers, and it burns gills at concentrations a test kit can barely see. It is more dangerous in hot or alkaline water, so be quicker to act in summer heat or when pH runs above 8. In a settled, cycled setup it should always read 0.
0: perfect. This is the baseline, not an achievement, but it is the right baseline.
A trace, up to about 0.25: the biofilter is falling behind the feeding. Do a partial water change, feed lightly or not at all, and retest tomorrow. If it keeps showing up, the setup is overstocked, overfed, or not fully cycled yet.
Above 0.25: act today. Large water change with dechlorinated, temperature-matched water, skip feeding, retest in a few hours, and repeat until it holds near zero. Fish sitting in real ammonia do not have weeks.
Nitrite: zero, and the cycle is not done until it stays there
Nitrite is what ammonia becomes on its way to nitrate, and it is nearly as nasty, blocking fish blood from carrying oxygen. Any nitrite reading means the nitrogen cycle is still finishing or was recently disrupted.
0: exactly right.
A trace, up to about 0.25: partial water change, light feeding, daily retests until it holds at zero. This is normal in a tank's first weeks.
Above 0.25: genuinely dangerous. Large dechlorinated water change today, and keep testing daily until it reads zero and stays there.
Nitrate: the relaxed one
Nitrate is the end of the line, the mild leftover the cycle produces on purpose, and medaka shrug at it in moderation. It only becomes a problem when it quietly accumulates for months.
Under 40: fine. Carry on.
40 to 80: elevated, not an emergency. Tighten your water-change rhythm, and put floating plants to work; they eat nitrate for a living.
Over 80: high. Bring it down with a series of partial changes over several days rather than one enormous change. Fish adapt to gradually improving water far better than to a sudden swing, even a swing in the right direction.
pH: the number people worry about most and need to fix least
Medaka are comfortable from roughly 6.5 to 8.5, which covers nearly every tap in the country. A steady 8.0 beats a perfect 7.0 that bounces around.
6.5 to 8.5: leave it alone. Truly. The chemicals sold to nudge pH cause more disasters than the numbers they chase.
Below 6.5: the water is acidic, and shells and stability both suffer down there. Low KH is the usual culprit, and a small amount of crushed coral raises it gently.
Above 8.5: medaka tolerate a lot, but confirm it is steady rather than swinging daily. If it swings, test KH before blaming anything else.
Temperature: read it with the season in mind
60s and 70s°F: the comfortable range where medaka eat and color up, and egg laying picks up as the water warms.
Below about 50°F: cold water. They slow down and eat very little, so stop feeding when they stop eating; uneaten food becomes ammonia. My winterizing guide covers the cold months properly.
78 to 85°F: warm. Fine for stretches, but warm water holds less oxygen, so add shade and gentle surface movement during a heat wave.
Above 85°F: watch closely. The fish can take it, but oxygen gets scarce in still water, especially overnight, so shade the container and get the surface moving. Warm water suffocates fish before it cooks them.
KH and GH: the two nobody tests until something is weird
KH (carbonate hardness) is the buffer that holds pH steady. Below about 3 degrees of KH (roughly 54 ppm), pH can swing and even crash overnight. A little crushed coral in the flow raises KH slowly, and only while the water is on the acidic side, so it is a gentle safety net rather than a cure for alkaline water.
GH (general hardness) is dissolved minerals. Medaka handle soft water, but snails build their shells from GH, so below about 4 degrees of GH (roughly 70 ppm) you will eventually see pitted, thinning shells. Moderate hardness keeps everyone comfortable.
What I actually do with a bad test
The same three moves, in order, nearly every time. First, a partial water change with dechlorinated, temperature-matched water, because it helps almost everything and harms almost nothing. Second, less food, because feeding is the tap that fills the ammonia bucket. Third, retest tomorrow instead of adding a product tonight. Most water problems are solved by water, patience, and restraint, in that order.
And if the numbers are fine but the fish still look wrong, the water was not the story: the fish health guide picks up from there.
Common questions
What should my ammonia level be for medaka?
Zero. Ammonia is the one reading with no acceptable amount: in a settled setup it should always test at 0. A trace up to about 0.25 means the biofilter is falling behind, so do a partial water change and retest the next day. Anything above that calls for a large water change today, with dechlorinated, temperature-matched water, and no feeding until it reads zero again.
What is a safe nitrate level for medaka?
Under about 40 ppm (as nitrate, the way most hobby kits report it) is comfortable for medaka. Between 40 and 80 is elevated but not an emergency: tighten your water-change rhythm and let floating plants absorb the excess. Over 80, bring it down with a series of partial changes over several days rather than one huge change, because fish handle gradual improvement far better than a sudden swing.
My ammonia and nitrite are both high. Which do I fix first?
They share the same fix, so you do not have to choose: a large water change with dechlorinated, temperature-matched water helps both immediately. Both readings above zero at once almost always means the tank is still cycling or the biofilter was recently disrupted, so ease off feeding and test daily until both hold at zero.
Do I need to test KH and GH?
Not routinely, but they explain problems the other tests cannot. KH below about 3 degrees (roughly 54 ppm) lets pH swing and crash, and a little crushed coral raises it slowly. GH below about 4 degrees (roughly 70 ppm) means soft water, which medaka tolerate but snail shells do not, since shells are built from dissolved minerals. If your pH is unstable or your snail shells are pitting, these two tests hold the answer.