For a typical domestic guppy aquarium, useful working ranges are around 22–26°C, pH 7.0–8.0 and GH 8–20 dGH. Ammonia and nitrite should be undetectable. Keep nitrate low through appropriate maintenance, and check KH alongside pH stability rather than treating hardness as one single measurement.
These are starting guides, not numbers to chase with a different bottle every morning. A settled aquarium with suitable water is usually better served by understanding its normal readings than by making constant adjustments.
Start by testing your tap water
Test the source as well as the aquarium. Knowing the pH, GH, KH and nitrate of the water you add helps explain what happens after a water change.
Use conditioner suitable for the chlorine or chloramine in your supply. Check the instructions and dose for the volume being treated. Household softened water is not automatically suitable aquarium water; it may have had useful calcium and magnesium exchanged for sodium.
Write results down. “The water is fine” doesn't help you identify a change nearly as much as a short record of actual numbers.
pH: don't adjust a suitable tank unnecessarily
A pH around 7.0–8.0 is a useful range for domestic guppies. If your fish are settled in suitable water at 7.2, discovering a recommendation for 7.5 isn't a reason to start adding an adjuster.
A changing result deserves investigation. Check the source water, maintenance history and buffering before repeatedly pushing pH back up. A single slightly different reading and an ongoing downward trend are not the same problem.
If the water is far outside suitable conditions, work out a gradual, repeatable correction with knowledgeable help. Don't make a sudden large change simply to improve the appearance of the test result.
GH and KH do different jobs
GH describes general hardness, mainly calcium and magnesium. Around 8–20 dGH is a useful working range. Extremely soft water may need preparation to provide suitable mineral content.
KH gives an indication of buffering—the water's ability to resist changes in pH. A satisfactory GH result doesn't necessarily mean that buffering is adequate. Look at KH together with whether pH stays steady between water changes.
If remineralisation is needed, check what the product changes. Some raise GH; others affect GH and KH. Prepare replacement water consistently, following the instructions and testing it before use. Ordinary aquarium salt is not a substitute for calcium and magnesium.
Ammonia and nitrite need a different response
These are not readings to fine-tune towards a preferred number. Detectable ammonia or nitrite in an occupied aquarium calls for prompt investigation and corrective action.
Check filtration, recent stocking, feeding and anything that may have damaged the biological filter. Use appropriately conditioned, temperature-matched partial water changes as needed, retest and seek help if the readings persist or fish are distressed.
For a new tank, establish the biological filter before adding fish. The filter guide explains why installing equipment is not the same as cycling it.
Use nitrate to understand the maintenance load
Keep nitrate reasonably low; below roughly 40 mg/l is a useful general target, with lower levels preferable where achievable. It is not a boundary that guarantees safety below it or explains every problem above it.
Compare the tank with the supply. If tap water already contains substantial nitrate, water changes with that same supply have limits. Discuss an appropriate alternative water-preparation routine rather than making unplanned chemistry changes.
Stocking, feeding portions, plant growth and maintenance all affect the trend. Check temperature separately too: heater settings need verifying with a thermometer.
You don't need to become a chemist. You need to know what each result tells you and what, if anything, to do next. That is the approach I take throughout Guppies.
