How to Lower Alkalinity (KH) in a Fish Tank, and What It Does to pH

Check that your fish need lower KH

API says GH and KH “should match their natural habitat”, and its kit leaflet says: “When keeping a community aquarium with a variety of tropical fish, adjust the GH and KH to 3 – 6º (50-100 ppm).” The leaflet lists harder water for some fish, such as Rift Lake cichlids. Aqueon, an aquarium equipment maker, says it is best not to change pH unless you are breeding “sensitive or wild caught fish from environments with extremely low or high pH”, and: “Most fish will adapt as long as water quality is good and pH is stable.” KH and pH usually move together, so this applies to KH too (our reading).

Test KH and pH in the tank and in your tap water

If your hand goes into the tank for the sample, unplug first. Fluval’s AquaClear filter manual says: “CAUTION – Always unplug or disconnect all appliances in the aquarium from electricity supply before placing hands in water, before putting on or taking off parts and while the equipment is being installed, maintained or handled.” Plug every appliance you unplugged back in once your hand is out. For pH, follow your pH kit’s directions. API’s KH directions: fill a clean tube “with 5 ml of water to be tested”, add KH Test Solution “1 drop at a time”, “Cap test tube & invert several times after each drop”, and stop when the water “turns from blue to yellow”. API: “The KH value is determined by the number of drops of test solution that must be added to turn the water in the test tube bright yellow.” Its chart converts drops to °dKH and ppm. API’s safety data sheet for the KH Test Solution carries the signal word “DANGER” and “Causes severe skin burns and eye damage.” It says: “Wear protective gloves/protective clothing/eye protection/face protection.” For eyes: “IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing.” and “Immediately call a POISON CENTER or doctor/physician.” For skin: “IF ON SKIN (or hair): Remove/Take off immediately all contaminated clothing. Rinse skin with water/shower.” If swallowed: “IF SWALLOWED: Rinse mouth. Do NOT induce vomiting.” It also says “Store locked up.” For eye contact, its first-aid section says “Wash out immediately with water.” and “If irritation continues, seek medical attention.”, and on lenses it differs from the label: “Removal of contact lenses after an eye injury should only be undertaken by skilled personnel.” For swallowing it is milder (“Immediately give a glass of water.” and “First aid is not generally required.”); follow the label and call poison control (our reading). In the US, the National Capital Poison Center, a nonprofit, says to “call a poison center at 1-800-222-1222”. If the tank’s KH reads higher than the tap’s, something in the tank may be adding carbonate (our reading; step 3).

Take out rock and decor that add carbonate

Unplug all appliances first (Fluval’s caution, step 2). Aqueon says crushed coral and dolomite gravel are “calcium carbonate-based gravels” that “slowly dissolve over time, raising and buffering pH”. Swell UK, an aquarium retailer, names “rocks which contain lime, like Limestone, Tufa rock, or Ocean Rock.” None of the sources we read covers replacing gravel or other substrate in a running tank, so this guide covers rock and decor only (our reading). Once your hands are out, restart the filter first: for an AquaClear, Fluval’s manual says to “Fill filter case completely with water.”, then “Plug power cord into electrical outlet. Allow 30-45 seconds for priming.”, and warns: “IMPORTANT – DO NOT OPERATE FILTER WITHOUT WATER”. Other filters follow their own manual. Hydor, a heater maker, says its heater “can ONLY then be turned back on when the Aquarium is full and the heater is completely submerged.” Then plug every other appliance you unplugged back in.

Work out a blend of RO and tap water

Atlas Scientific, a maker of water-quality sensors: “The best way to lower alkalinity in aquariums is to dilute the water with purified water that has a low-mineral content. The easiest way to produce purified water is to use reverse osmosis.” Aqueon: “Always prepare water and test pH before adding it to your aquarium.” ForAquarium, a fishkeeping guide site, says “a mixture of 50% tap water and 50% RO water will reduce the total GH and KH by half”, and notes: “These estimates assume the RO water has a KH of 0 dKH.” The rule behind it: the new water’s KH is about the tap’s KH times the tap share, with RO at 0 (our arithmetic). The tank falls only part way with each partial change, so test after each one and do not blend below the first floor in step 8, 3 dKH for a community tank (our reading). ForAquarium adds that low-mineral water is “typically mixed with mineralized tap water or remineralized before use”; our softening guide covers remineralizers. Test the bucket’s KH and pH before it goes in.

Change water in partial steps

Switch off and unplug the heater first, as long ahead as its maker says (our sequencing). Hydor’s support page says “at least 30 minutes before any waterchange”; its Theo manual says “at least 5 minutes before taking it out of the water or before adding or changing the water”, and: “Always ensure the water level stays above the ‘Minimum water level’ mark shown on the heater”. With no manual, our reading is the longer wait. Unplug every other appliance when your hands go in (Fluval’s caution, step 2). API’s leaflet: “Partial water changes can be used to decrease KH.” Aqueon says: “Avoid water changes of more than 25% as larger water changes can stress your fish and upset the balance in your aquarium.” API says: “It is not recommended to do more than 30% because it may cause loss of bacteria and be stressful on the fish.” Seachem, in an Acid Buffer FAQ, says: “More sensitive tank inhabitants will do better with smaller, more frequent water changes and slower alterations of water parameters.” These are upper limits, not targets (our reading). Swell, after removing rocks, suggests “several large water changes”; the makers’ limits differ, so use the makers’ limits (our reading). As Aqueon says, “Be sure to match temperatures of the new water with the aquarium.” Once refilled, plug every appliance back in: restart the filter (prime an AquaClear as in step 3) and turn the heater on only when the tank is full and the heater is completely submerged (Hydor). Test KH and pH after each change; if your hand goes in the water, unplug all appliances first (step 2) and plug them back in after. No source we read gives a safe KH drop per day (our reading); Aqueon says that with fish in the tank, “never make sudden or drastic changes to pH or any other water parameters.”

Use peat after diluting, not instead

Sources differ on peat and KH. ForAquarium says peat moss “can gradually lower aquarium KH and pH by releasing organic acids into the water”, and Atlas says peat, driftwood or Indian almond leaves “absorb alkaline elements in the aquarium water, lowering the alkalinity, and pH.” TFH Magazine, a fishkeeping magazine, and Aqueon treat peat as a pH tool, and TFH says “in the presence of high alkalinity, the effect of any pH-lowering material will be quite fleeting”. TFH adds: “it is advisable to soften harder water beforehand, by adding RO water for example.” To try peat, unplug all appliances first (step 2). Aqueon says to put it in the filter: “Use a mesh media bag to keep it contained and use only peat products designed for aquariums.” ForAquarium suggests pre-soaking the moss “in a separate container of water for 24–48 hours” and: “Test the KH and pH before using the treated water during an aquarium water change.” TFH: “Dosing should be as per the manufacturer’s instructions.” TFH warns: “it is possible to overdose tannins unintentionally in the aquarium by using too many leaves or too much peat”, and leaves, peat and their extracts “should be dosed in moderation and with care.” Plug every appliance you unplugged back in, restarting the filter and heater as in step 3. Our guide to lowering aquarium pH covers peat, leaves and driftwood, with more of TFH’s and Aqueon’s cautions.

If you use a KH-lowering product, follow its label

If your hand goes in the water, unplug all appliances first (step 2) and plug them back in after. Aqueon says pH products “should be avoided altogether or used with extreme care by experienced aquarists to avoid sudden drastic pH shifts”. Seachem says its Acid Buffer “is intended to lower pH and KH in the aquarium”. Asked whether it suits a given tank, Seachem answers “Possibly”: “Acid Buffer™ is exclusively intended for freshwater tanks and not every type of aquarium fish is suited to the range of pHs which Acid Buffer™ can be used to create. Research your fish to know what pH they would prefer.” It also calls it its recommended buffer for planted tanks. Seachem says: “Each dose will decrease alkalinity by approximately 0.6 dKH”; follow the label for the amount. It adds: “Be aware that lowering the pH using Acid Buffer™ alone should be done with care to ensure that the pH does not fall too far.” Its overdose FAQ: “An overdose of Acid Buffer™ can result in a sudden and stressful shift in pH and KH.” It continues: “If pH or KH of the water is still incorrect despite repeated additions of a buffer like Acid Buffer™, it is likely that there is a high concentration of acids or buffers in the aquarium that are resisting the change in pH.” Test KH and pH before each dose, and stop at the first floor in step 8, 3 dKH for a community tank (our reading). Seachem’s label caution: “Causes serious eye irritation. Keep out of reach of children. Avoid contact with skin.” Its safety data sheet says more: signal word “Danger”, “Causes serious eye damage.” and “Harmful if swallowed.” It says: “Wear eye or face protection”, “Wash hands thoroughly after handling”, “IF SWALLOWED: rinse mouth. Do NOT induce vomiting.”, “IF ON SKIN: Remove all contaminated clothing. Rinse skin with water.” and “IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing. Immediately call a POISON CENTER or physician.” Its first-aid section differs: “Immediately flush eyes thoroughly with water for 15-20 minutes. Remove contact lenses after the initial 1-2 minutes and continue flushing for several additional minutes. If effects occur, consult a physician, preferably an ophthalmologist.” It also says to “Avoid generating and breathing dusts or particulates generated by this product.”

Retest, and stop before KH runs too low

If your hand goes in the water, unplug all appliances first (step 2) and plug them back in after. Tetra, a water-care maker, says: “With regard to pH stability, the KH should not generally drop below 3 °dH”. API’s leaflet says: “Some tap water supplies have very low KH (below 3 ºdKH) which can contribute to wide pH swings in the aquarium.” UF/IFAS, on aquatic systems: “At a total alkalinity less than 20 milligrams per liter (mg/L), common species of nitrifying bacteria do not function effectively.” That is about 1.1 dKH (our arithmetic, from ForAquarium’s “One dKH is equivalent to approximately 17.848 mg/L”, assuming both are as calcium carbonate). UF/IFAS adds that “once alkalinity is depleted, pH of the water can drop significantly and quickly”, which it says can kill fish and nitrifying bacteria. For a community tank, treat 3 dKH as the first floor (Tetra, API); UF/IFAS’s figure, about 1.1 dKH, is where nitrifying bacteria stop working well (our reading). If ammonia reads above zero, see our guide to lowering ammonia. Before adding fish, see our guide to adding new fish.