Understanding Your Fish Tank Water Hardness

            We have all seen the instructions on testing kits that tell us how to test for certain chemicals in the tank but sometimes we are not exactly sure what we are testing and why. In this article I will cover the two basic types of water hardness which are General Hardness (GH) and Carbonate Hardness (KH).

            General Hardness (GH) is a test that measures the amount of Calcium (Ca²⁺) and Magnesium (Mg²⁺) that has dissolved in your freshwater tank. Hard water, testing at > 200 ppm, is high in both Calcium and Magnesium. Soft water, testing between 50-100 ppm is low in these minerals. It is important to know that a special saltwater test is required for saltwater aquariums because the ions of Calcium and Magnesium are higher than the testing ranges for most test.

            The general hardness of your tank can increase over time and so it is important to check the water level continuously. As water evaporates it leaves behind the metal elements which increase the general hardness, when water is added more metals are added to the tank further increasing the hardness. Adjusting the level of your water hardness can be as simple as completing partial water changes using deionized water or adding a water softener “pillow” to the filter to reduce the hardness, or by using specialized salt to increase the hardness.

            Different types of fish require certain hardness levels and the following is small listing of fish and some plants along with their hardness requirements.

30ppm- Discus, arowanas, elephant nose, neons and cardinals.

60ppm- Certain Tropical fish, angelfish, tetras, botia, and community aquariums.

120ppm- Certain Tropical fish, swordtails, guppies, mollies, cichlids and certain goldfish.

180ppm- African Cichlids and certain goldfish.

            Carbonate Hardness (KH) is also known as Alkalinity and is the measure of Carbonate (CO₃²) and Bicarbonate (HCO₃). This type of hardness helps to stabilize the pH in your aquarium and with a high level (> 200ppm) your aquarium will have a high pH. In low levels, your tank water will be acidic and you will experience rapid shifts in pH. Carbonate is used by aquarium plants and consumed in the filter so this level will need to be maintained or it will fall and become unstable.

           Reducing you Carbonate Hardness is as simple as doing a partial water change and using distilled or deionized water. Increasing can be done by using a pH adjuster

            The following are acceptable levels for the listed fish. Please check the levels for your individual fish.

40ppm- Discus, arowanas, elephant nose, neons and cardinals.

80ppm- Certain Tropical fish, angelfish, tetras, botia, and community aquariums.

120ppm- Certain Tropical fish, swordtails, guppies, mollies, cichlids and certain goldfish.

180ppm- African Cichlids, certain goldfish brackish water and certain marine fish.

240ppm- Rift Lake Cichlids, certain goldfish, brackish water and certain marine fish.

Rasboras Profile

• Should be kept in a tank of at least 20 gallons. These fish range in size but they usually are around 2-3 inches.
• They are peaceful.
• Are Omnivores.

Water conditions should be:      Temperature: between 73 and 79 F

                                                KH: 2-5

                                                pH: 6.0-7.5

Rasboras Compatibility

Caution should be used when placing Rasboras with New World Cichlids, Larger Catfish, Sharks and Invertebrates.

 Extreme Caution:  Rasboras should not be placed with Malawian, Tanganyikan or African Cichlids, Fancy Goldfish or Brackishfish.

Rainbowfish Profile

• Should be kept in a tank of at least 50 gallons. These fish range in size but they usually are around 3-4 inches.
• They are peaceful.
• Are Omnivores.

Water conditions should be:      Temperature: between 70 and 77 F

                                                KH: 10-12

                                                pH: 7.0-7.5

Rainbowfish Compatibility

Caution should be used when placing Rainbowfish with New World Cichlids, Bettas, Hatchets, Larger Catfish, Invertebrates and Brackishfish.

 Extreme Caution:  Rainbowfish should not be placed with Malawian, Tanganyikan or African Cichlids and Fancy Goldfish

Plecos Profile

• Should be kept in a tank of at least 30 gallons. These fish range in size but they usually are around 3 inches some types will get very large so be prepared to move them if need be.
• They are peaceful.
• Are Omnivores.

Water conditions should be:      Temperature: between 72 and 76 F

                                                KH: 8-12

                                                pH: 6.5-7.5

Plecos Compatibility

Caution should be used when placing Plecos with Discus, Larger Catfish, Invertebrates and brackishfish.

 Extreme Caution:  Plecos are good tanks guests for most species.

Platys Profile

• Should be kept in a tank of at least 10 gallons. These fish range in size but they usually are around 3 inches.
• They are generally peaceful.
• Are Omnivores.

Water conditions should be:      Temperature: between 64 and 77 F

                                                KH: 10-25

                                                pH: 7.0-8.2

Platys are generally a hearty and forgiving species making them perfect for new aquarist.

Platys Compatibility

Caution should be used when placing Platys with New World Cichlids, Barbs, Larger Catfish, Loaches, Sharks, Suckermouth, Catfish and Invertebrates.

 Extreme Caution:  Platy should not be placed in a tank with Malawian, Tanganyikan or African cichlids, Fancy Goldfish and Brackish Fish.

Mollies Profile

• Should be kept in a tank of at least 20 gallons. These fish range in size but they usually are around 5 inches.
• They are generally peaceful.
• Are Omnivores.

Water conditions should be:      Temperature: between 68 and 82 F

                                                KH: 1-2

                                                pH: 7.0-8.0

Mollies are generally a hearty and forgiving species making them perfect for new aquarist.

Mollies Compatibility

Caution should be used when placing Mollies with New World Cichlids, Larger Catfish, Loaches, Sharks, Suckermouth, Catfish and Invertebrates.

 Extreme Caution:  Mollies should not be placed in a tank with Malawian, Tanganyikan or African cichlids and Fancy Goldfish.

Loaches Profile

• Should be kept in a tank of at least 50 gallons. These fish range in size but they usually are around 3 inches.
• They are generally peaceful.
• Are Omnivores.

Water conditions should be:      Temperature: between 72 and 86 F

                                                KH: 8-12

                                                pH: 6.0-7.5

Loaches Compatibility

Caution should be used when placing Loaches with All cichlids, Fancy Goldfish, Guppies, Killifish, Large Catfish, Mollies, Platys, Swordtails, Invertebrates and Brackishfish.

 Extreme Caution:  Loaches get along with most species of fish and we do not have any information on fish that should not be put with them.

Larger Catfish Profile

• Should be kept in a tank of at least 120 gallons. These fish range in size but they can reach over 2 with some passing 3 feet.
• They are peaceful to aggressive.
• Are Omnivores.

Water conditions should be:      Temperature: between 75 and 81 F

                                                KH: 10-12

                                                pH: 6.8-7.3

Larger Catfish Compatibility

Caution should be used when placing Larger Catfish with All cichlids, Angelfish, Barbs, Bettas, Cory Cats, Danios, Discus, Gouramis, Guppies, Hatchets, Killifish, other Larger Catfish, Loaches, Mollies, Platys, Plecos, Rainbowfish, Rasboras, Sharks, Suckermouth Catfish and Brackishfish.

Extreme Caution:  Larger Catfish should not be placed in a tank with Fancy Goldfish and Tetras.

Killifish Profile

• Should be kept in a tank of at least 20 gallons. These fish range in size but they will usually reach 2-3 inches.
• They are peaceful to semi-aggressive.
• Are Carnivores.

Water conditions should be:      Temperature: between 73 and 79 F

                                                KH: 5-8

                                                pH: 6.0-7.5

Killifish are a freshwater fish easily recognized by its unique tail and varied appearance.

Killifish Compatibility

Caution should be used when placing Killifish with New World Cichlids, Barbs, Bettas, Larger Catfish, Sharks, Invertebrates, and Loaches.

Extreme Caution:  Killifish should not be placed in a tank with Malawian, Tanganyikan or African Cichlids, Fancy Goldfish and Brackish fish.

Hatchets Profile

• Should be kept in a tank of at least 10 gallons. These fish range in size but they will usually reach 2-3 inches.
• They are peaceful.
• Are omnivores.

Water conditions should be:      Temperature: between 75 and 81 F

                                                KH: 10-18

                                                pH: 5.5-7.5

Hatchets are a freshwater fish easily recognized by its “pot-belly” appearance.

Hatchets Compatibility

Caution should be used when placing Hatchets with New World Cichlids, Barbs, Larger Catfish, Rainbowfish, Sharks, and Invertebrates.

Extreme Caution:  Hatchets should not be placed in a tank with Malawian, Tanganyikan or African Cichlids, Fancy Goldfish and Brackish fish.