• Understanding Compatibility,
• Food types,
• Water chemistry,
• Species behavior,
• Environmental.

Water chemistry is a critical factor in maintaining a healthy and stable environment in aquariums. It involves understanding and managing various chemical parameters of the water.  Since an aquarium is a defined space, it is essential that all occupants in an aquarium be able to tolerate and hopefully thrive in the same aquatic biome.

To better understand salinity and water hardness the Water Chemistry chart allows the aquarist to plot an aquarium occupant’s preferences.  In this area of compatibility all occupants should be closely plotted together.  A potential new addition would need to be in the same quadrant as the other occupants in the aquarium for there ti be any hope of compatibility.

In this plotting chart salinity is presented as a gradient from zero parts per thousand (PPT) at the bottom up through 40 ppt at the top.  Anything from 0 to 1 ppt is considered freshwater.  Some scientists argue for an upper limit of 0.5 ppt as the upper limit of salinity to freshwater.  Saltwater is anything above 30 ppt.

Anything between 30 ppt and 1ppt is considered brackish water.  Within brackish water there are three levels of low, medium, high.  Their ppt designations vary considerably.  Consequently, those segregations are not presented in the compatibility chart.

An aquarium occupant will exist somewhere in the spectrum of 0 ppt to 40 ppt.  Depending on the aquarium, the water chemistry salinity will match with the potential new occupant.

Salinity of water: Another element of water chemistry is the salinity value of the water. Salinity levels, which refer to the concentration of dissolved salts in water, can have a significant impact on aquarium fish, particularly for those kept in marine or brackish water setups.

Different fish species, and other aquarium occupants, have varying tolerance levels for salinity.  Maintaining the appropriate water salinity is essential for their health and well-being.  There are three different classifications used to describe the salinity of aquarium water, and naturally present water in the wild.  They are Freshwater, Brackish water, and Marine water (saltwater).

The exact amount of dissolved salts that each classification uses as a definition varies slightly (.5 ppt), but generally there is a fairly firm consensus on what constitutes freshwater and salt water (marine).

Freshwater vs. Marine vs. Brackish Fish:

• Freshwater Fish: Freshwater fish come from environments with low salinity levels, typically less than 1 part per thousand (ppt) of dissolved salts. They are adapted to this low-salinity environment and can’t tolerate high salinity levels.
• Marine Fish: Marine fish are adapted to high salinity levels, usually ranging from 30 to 40 ppt or even higher, depending on the species. They have specialized salt-regulating mechanisms to maintain their internal salt balance in seawater.
• Brackish Fish: Brackish water fish inhabit estuaries or areas with varying salinity levels, typically ranging from 1 to 30 ppt. They can adapt to a wide range of salinities but often have preferred salinity levels within this range.

The overarching difference between aquarium occupants and their ability to handle dissolved salts is a biochemical process called osmoregulation.  Osmoregulation is a vital physiological process that enables organisms to survive and thrive in a wide range of environments by maintaining the balance of water and solutes in their bodies.  Osmoregulation mechanisms can vary widely among different organisms, depending on their habitat.

Osmoregulation:

• Marine Fish: Marine fish, which live in high-salinity environments, actively pump excess salts out of their bodies through specialized cells in the gills, while conserving water and actively transporting ions like sodium and chloride into their tissues.
• Freshwater Fish: Freshwater fish, which live in low-salinity environments, actively take in salts through their gills and excrete large amounts of dilute urine to prevent water loss.
• Marine Invertebrates: Many marine invertebrates, including crustaceans and mollusks, have specialized structures like gills, nephridia, or specialized cells to manage osmoregulation according to their specific needs.

Compatible aquarium occupants will have the same or similar salinity tolerance.  The impact of placing an occupant incorrectly in a high salinity or freshwater aquarium can be deadly.  Consider the following situations:

• Freshwater Fish in High Salinity: If freshwater fish are exposed to high salinity levels, they will struggle to expel excess salts, leading to dehydration and stress. Prolonged exposure can be fatal.
• Marine Fish in Low Salinity: Marine fish placed in freshwater or brackish water may have difficulty maintaining their internal salt balance, leading to stress, organ dysfunction, and eventually death.
• Brackish Fish in Inappropriate Salinity: Brackish water fish may adapt to different salinity levels, but abrupt changes can stress them. It’s essential to gradually acclimate them to new salinity conditions.

Water hardness, specifically the measurement of dissolved minerals, can significantly impact the health and well-being of aquarium occupants, particularly fish and invertebrates. The two main aspects of water hardness are general hardness (GH) and carbonate hardness (KH).

For the purposes of species compatibility analysis, the GH or degree of GH(dGH) is used in this guide. Carbonate hardness (KH) relates more towards the stabilizing effect Carbonate and bicarbonate ions have against pH variations in the aquarium water.

Some aquatic occupants are highly sensitive towards changes in pH. And that sensitivity is addressed under pH.

For more information regarding water hardness, visit:  SNIPPET: What is Hard Water.

dGH (Degree of General Hardness):

• Calcium and Magnesium Content: GH primarily measures the concentration of calcium and magnesium ions in the water. These minerals are essential for the growth and overall health of fish and invertebrates. A lack of calcium and magnesium can lead to various health issues.
• Impacts on Fish: In soft water (low GH), fish that require higher levels of calcium and magnesium may suffer from deformities, weakened bones, and poor growth. Conversely, in hard water (high GH), fish adapted to soft water conditions may struggle with stress and reduced reproductive success.
• Invertebrates: Many aquatic invertebrates, such as snails and crustaceans, rely on calcium to build and maintain their shells or exoskeletons. In soft water with low GH, these organisms may have difficulty forming and maintaining their protective structures.

dGH Impact on Specific Species:

• Species Variability: Different fish and invertebrate species have varying preferences for water hardness. It’s crucial to research and understand the specific requirements of the species in your aquarium. Some species are adaptable and can tolerate a range of hardness levels, while others are highly specialized.
• Breeding and Reproduction: Water hardness can influence the breeding and reproduction of fish. Some species require specific hardness levels to trigger spawning behavior or to raise healthy fry.

Acclimation: When introducing new fish or invertebrates to an aquarium, it’s essential to acclimate them gradually to the water’s hardness and other parameters. Sudden changes in water hardness can stress or harm aquatic organisms.

The degree of hardness of the aquarium water is represented in the Water Chemistry compatibility chart across the chart from left to right.  The dGH is zero at the far left and rises  to 17 dGH at the far right.  Aquarium occupants will have a preference for water hardness, and that preference should be reflected in the aquarium water chemistry.  For compatibility purposes, all aquarium occupants should be within 4 points of one another.

In addition to salinity and water hardness, pH and water temperature have crucial roles in determining aquarium occupant compatibility.  The Water Chemistry 2 chart depicts two opposing scales pf pH and Water temperature.

The pH scale (described below in detail) is represented along the vertical axis with 7.0 pH centered on the vertical axis.  The entire pH scale is not presented.  Rather, a portion of the scale from 5.0 – 9.0 is presented.  This portion of the pH scale is where most aquatic species exist.

Along the bottom of the chart is water temperature scale.  The scale represented is from 60 F – 90 F.  This temperature range is the most common range for aquariums.  However, if the aquarist is configuring a specialty tank for occupants of colder climates, the range would need to be adjusted to reflect a different temperature range.  The median temperature shown is 75 F.

To use this chart the aquarist would plot their existing aquarium occupants’ pH preferences and their optimum temperature.  The intersection of the two components would represent the existing occupants tolerances.  A new occupant would likewise be plotted.  The representative plots will most likely be lines, as there is usually a range of tolerated pH and temperatures that aquarium animals can tolerate.

To determine compatibility the aquarist would look for intersecting lines between the existing occupants and the proposed new occupants.  If the lines do not intersect or are a distance apart, then the likelihood of achieving good compatibility between occupants is low.

The pH scale ranges from 0 to 14, with 7 being neutral. A pH value below 7 indicates acidic conditions, while a pH value above 7 indicates alkaline or basic conditions. The scale is logarithmic, meaning that each whole pH unit represents a tenfold difference in acidity or alkalinity. For example, water with a pH of 6 is ten times more acidic than water with a pH of 7.  Most occupants and fauna in the wild exist within a point, or at most  two points, of 7.0 pH.

It is important to select potential aquarium occupants that share the same pH tolerance.  Many aquatic creatures can survive in slight variations from their preferred pH environment, but will not thrive.  If the aquarist wants to breed their occupants and otherwise enjoy long healthy lifespans of their aquarium occupants, it is critical that pH be closely monitored and adjusted appropriately to maintain the desired pH level.

pH also contributes to overall aquarium health and acts as a limiting agent against algal blooms, bacterial blooms, and many water-borne pathogens.

The Water Chemistry quad graph depicts pH along the left axis and has a range of 5.0 pH to 9.0 pH.  This allows for 2 points on either side of neutral 7.0 pH.  To be compatible an aquarium occupant should match other occupants as close as possible.  If the new occupant is outside the dotted ring (represents a full point on the pH scale either side of 7.0 pH), the aquarist should reconsider their occupant selection.

Similar to pH, temperature of the water is equally important to compatibility among aquarium occupants.  The typical range for aquarium fish is 70 F – 80 F.  However, there are some species that prefer much cooler temperatures in the 50 F – 60 F range.

Combining aquarium occupants with large ambient temperature variances is not recommended.  For compatibility, occupants should be within 5 degrees of their preferred temperature. It is possible for occupants to survive larger variances up to 10 or more degrees.

Keep in mind that large temperature differences between occupants will have a stressful effect.  With increased stress comes a general inclination towards poor health and the inability of the aquarium occupant to fight off pathogens.

In the aquarium biome, compatibility is largely comprised of four areas.  They are: Food types, Water chemistry, Species behavior, and Environmental.   The closer there is alignment in these areas and their sub-elements, the greater the compatibility potential.  Many aquatic occupants will overlap in these areas, or not be specific to a particular element in an area.