Article: Preparing water for freshwater aquariums


1. Introduction & The Basics

2. Water Parameters

2a. Chlorine and Chloramine 2b. Ammonia
2c. pH 2d. Alkalinity (KH)
2e. GH 2f. Trace elements & Heavy Metals

3. Preparing water

3a. Chlorine and Chloramine 3b. Ammonia
3c. pH 3d. Alkalinity (KH)
3e. GH 3f. Trace elements & Heavy Metals



Proper water preparation is vital for aquarium maintenance, and successful fish keepers can attribute a notable part of their success to determining a good method of water preparation for their aquarium and sticking with it consistently.  In this article, we'll be covering the methods of preparing water for freshwater aquaria, from beginner to advanced.

The Basics

For many fish keepers, preparing water for the aquarium is as simple as adding Prime® to their tap water.  Prime® is dosed at a rate of 5 mL per 50 gallons or 2 drops per gallon, and can either be added to the new water first (according to the volume of new water) or directly to the the aquarium if filling with a hose (dose according to the full volume of the tank - we recommend to split the dose so that half is added before the new water and half is added after).  

This simple method of water preparation works well if local tap water parameters are not too far from the fish's natural preferred range, and the fish being kept are hardy and tolerant of fluctuating water parameters.  However, some fish species are more sensitive than others, and local tap water sometimes contains buffers, minerals, or contaminants that make it unsuitable for keeping your desired species of fish.  If this is the case, you will need to take steps to test and alter these water parameters to make your water suitable for your fish.

Water Parameter Overview

There are several major water parameters to keep in mind when preparing water for use in the aquarium.  Each of these is important for different reasons, although some will require more "upkeep" over time than others.  

Chlorine and Chloramine

These chemicals are added to tap water to kill harmful bacteria that can cause disease in humans.  Unfortunately, they will also kill beneficial bacteria colonies and fish in aquariums, so they must be removed from water before it is added to the tank.  Seachem recommends the use of a dechlorinator like Prime® to immediately remove chlorine and chloramine from tap water.


Ammonia in the aquarium can come from fish urine or from decomposing organic waste, but when preparing water for the aquarium, the most likely source of ammonia is chloramine.  When chloramine is broken down, ammonia is produced as a byproduct (this is unavoidable - anything that breaks down chloramine will inherently produce ammonia as a byproduct).  If the source water contains chloramine or ammonia, Seachem recommends to either use a separate ammonia detoxifier like AmGuard™ or a combination conditioner/detoxifier like Prime®.


pH is a measure of how Acidic or Alkaline the water is.  It's reported on a scale of 0 to 14, with 7.0 being neutral.  Higher numbers are alkaline while lower numbers are acidic.  Lots of factors in the aquarium and tap water will impact the pH, so while pH is an important water parameter, it can be helpful to think of pH as a result of other water parameters (like alkalinity) than as something to be directly altered.

Alkalinity (KH)

This is a measure of how resistant the pH is to change, and is a result of how much buffers are dissolved in your water.  Buffers are natural compounds that alter and maintain the pH.  There are many types of buffers, but the most common in tap water are phosphate and carbonate.  If the pH is high and is difficult to change, it is likely that the alkalinity is high.


This stands for "general hardness" and is a measure of the amount of minerals that are dissolved in water, particularly calcium and magnesium.  In nature, the same materials that leach buffers into water will also leach minerals in the water, so GH and alkalinity usually move together.  However, these water parameters do not actually impact each other (the GH can be high while the alkalinity is low and vice versa), so in the aquarium it is important to test and alter them separately.

Trace Elements and Heavy Metals

Fish absorb many types of essential trace elements directly from their water, so it is important to ensure that water has sufficient amounts of trace elements to support the fish.  Tap water often already contains plenty of trace elements, but they are utilized rapidly and so it is still recommended to dose a trace element between water changes, particularly in the case of large or fast-growing fish.

The unfortunate downside to tap water is that sometimes it contains harmful heavy metals, or even beneficial trace elements that have built up to the point of becoming toxic to fish.  Copper is notable among these for being commonly used in the plumbing of older houses.  Copper is an essential trace element when present at low levels, but is harmful to fish and invertebrates when present at high levels.  If there is reason to believe that water may contain toxic levels of metals, it is recommended to either use a metal-absorbing rising like CupriSorb™ or to swap to RO/DI water* instead of continuing to use tap water.

*RO/DI water is water that has been stripped of all minerals, buffers, and contaminants.  This type of water requires special preparation, which we will cover in "Altering Water Parameters"

Altering Water Parameters

Chlorine and Chloramine

Removing chlorine and chloramine is as simple as adding a water conditioner.  Prime® is the most popular water conditioner by Seachem®, but several other products will also remove chlorine and chloramine

Products Which Remove Chlorine & Chloramine

Prime® Safe® AmGuard™ StressGuard™
Neutral Regulator® Betta Basics™ Gold Basics™  
aquavitro alpha® aquavitro premier™ aquavitro shrimp start™  



If your tap water contains ammonia, it is likely that this contaminant is a byproduct of chloramine.  Seachem® water conditioners are designed to detoxify standard tap water levels of ammonia long enough so that beneficial bacteria colonies in your aquarium can break down and remove incoming ammonia.  If your incoming ammonia levels are so high that they threaten fish and cannot be detoxified using Prime® (>1.0 mg/L), we recommend seeking out a new water source such as RO/DI water.


As mentioned, it is typically more helpful to think of pH as a result of other water parameters rather than a parameter that needs to be directly altered.  Specifically, pH is primarily a result of the balance between buffers and acid sources. If the buffers are stronger or more concentrated, they will be able to raise the pH.  If acid sources are stronger or more concentrated, they will be able to lower the pH.  Here are some common causes of pH issues:

High pH

  1. Your alkalinity (KH) is too high. High alkalinity will tend to drag your pH upward unless your buffers are specially formulated to target a low pH.  Tap water often has a high concentration of buffers that target a high pH.  The best solution to this is to either dose an acid source like Acid Buffer™ to counteract the buffers or dilute the buffers using RO/DI water.
  2. Your buffers target a pH that is too high. This is common in tap water where carbonate is often present in high concentrations. Carbonate-based buffers target a pH of 7.4 or higher, and must be balanced by an acid source in order to maintain a neutral or acidic pH.  It is recommended to dose an acid source like Acid Buffer™ until the pH is near the desired range, and then add a suitable buffer to maintain the pH.  Check the “Alkalinity” section for suggested freshwater buffers.
  3. Your acid content is too low. Sometimes the use of a buffer that targets a high pH is unavoidable, and the fishkeeper must use an acid source (like CO2) to alter and maintain the ideal pH.  If there is little to no incoming acid source (such as a mixing bucket or empty tank) the equilibrium of the system will eventually swing toward the “target pH” of the buffer.  In this case, the fishkeeper should either lower the alkalinity or raise the acid content of the aquarium. 

Low pH

  1. Your alkalinity is too low. If the concentration and strength of your buffers is not high enough, then any acids in your water will be able to lower the pH.  Dose a buffer (suggestions in “Alkalinity”) to raise the alkalinity.
  2. Your buffers target a pH that is too low. This is very uncommon except in situations where the fishkeeper is intentionally dosing a buffer.  Swap to a buffer that targets a higher pH (suggestions in “Alkalinity”).
  3. Your acid content is too high. This is very common in established aquariums, which accumulate acid from fish waste and decomposing organic matter over time.  Either dilute the acids with a water change or dose a buffer (suggestions in “Alkalinity”) to counteract these acids.

Alkalinity (KH)

This is a measure of how resistant the pH is to change, and is a result of how much buffers are dissolved in your water.  Alkalinity in freshwater is fairly straightforward: if the alkalinity is too low, add more buffers (be aware that raising the alkalinity has the potential to raise the pH).  If it is too high, either dilute the buffers with RO/DI water or add an acid source (be aware that lowering the alkalinity has the potential to lower the pH).  Check out the guide to buffers below for help choosing a proper buffer for your system:



This stands for "general hardness" and is a measure of the amount of minerals that are dissolved in water, particularly calcium and magnesium.  While the amount of minerals in water is incredibly easy to raise (just add more minerals), lowering the GH is more difficult.  There are three major approaches to lowering GH in water preparation:

  1. Dilute the minerals with RO/DI water. This is Seachem®’s preferred method of reducing GH, but this does have the downside of diluting the buffers (alkalinity) and typically reducing the pH as well.  It may be necessary to dose a buffer after diluting the GH to keep the pH steady.
  2. Remove the minerals from the water through precipitation. Phosphate-based buffers can reduce GH by literally pushing the minerals out of solution.  This precipitation is visible in the water and is removed by fine filter media. This method is best when only a slight reduction in GH is necessary, as a large reduction will cause significant visible precipitation.
  3. Absorb the minerals from the water using a water softener pillow. Seachem® does not recommend this method, for the simple fact that water softener pillows work by replacing minerals with sodium.  The exact same fish that are sensitive to high GH tend to be sensitive to high sodium as well, so use of a water softener tends to simply replace one problem with another problem.

While reducing GH is difficult, raising GH is very simple.  Check out the guide below for an overview of the Seachem® mineral supplements.


Trace Elements and Heavy Metals

Trace elements are essential for proper fish health, and fish tanks often do not get enough water changes to keep up with the trace element needs of the fish and invertebrates.  It is recommended to add a trace element supplement like Fresh Trace™ once or twice per week to ensure that fish have enough trace elements to suit their needs.

Some tap water contains levels of trace elements that are too high for proper fish health, or may even contain dangerous heavy metals.  If this is the case, it is recommended to swap to RO/DI water rather than attempting to address the issue through supplementation.  Small buildups of heavy metals can be detoxified using water conditioners like Prime®, but large buildups are best addressed by direct removal.

RO/DI Water

Some fish keepers find that their tap water parameters are so extreme that they cannot be effectively altered using supplements, and will choose to swap to RO/DI water instead.  This is Seachem®'s preferred method of keeping fish, but does require a different approach to water preparation than using tap water.  RO/DI water is water that has been stripped of minerals, buffers, trace elements, metals, chemicals, contaminants, and anything else that is not pure H2O.  Fish require proper levels of minerals, buffers, and trace elements in their water, so when RO/DI water is being used, it is essential to use supplements to attain the proper pH, GH, KH, and trace element concentration for the fish.  See the sections above for recommended alkalinity, GH, and trace element supplements.




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