Aquarium Water Chemistry

Updated August 12, 2019
Author: Mike - FishLore Admin
Social Media: FishLore on Social Media

Anyone wanting to be successful at the tropical fish and aquarium hobby must put forth the time necessary to understand some basic fish tank water chemistry. Understanding aquarium water chemistry will help your fish to not only survive but thrive!

I recommend that you get a good aquarium water testing kit or many individual kits. You will need kits that will test for the following:

Aerobic Bacteria
This is bacteria that requires oxygen to live.

Anaerobic Bacteria
This is bacteria that can live without the presence of oxygen, or bacteria that does not require oxygen.

This chemical is the result of fish waste and decomposing food in the aquarium. Ammonia is the leading killer of tropical fish. New tanks that are going through the aquarium cycle or heavily stocked tanks will show ammonia readings with your test kits. Ideally, we want the ammonia reading to be 0 ppm.


Chloramine is a combination of chlorine and ammonia. It is a stronger disinfectant than chlorine alone and is used in areas where this extra disinfectant is needed. As with chlorine, you must eliminate this chemical from your tap water before adding it to your aquarium or it too will kill your tropical fish.

This chemical is found in most tap water and it is used to kill the bad bacteria in our drinking water. Clorine must be eliminated before entering your aquarium or it will kill your tropical fish.

This heavy metal can come in with the tap water if you have older copper pipes. It can also get introduced to your tank if you've used any copper based medications. Copper can be very harmful to fish and invertebrates.

Nitrites are converted to nitrates during the cycling process. Nitrates are not as toxic as ammonia or nitrites but they are harmful and will stress your fish at high enough levels. The only way to remove the nitrates is through a partial water change. Ideally you want to have test kit readings of less than 20 ppm in freshwater tanks and even less in saltwater tanks.

Ammonia gets converted to nitrite by the bacteria in your tank. Nitrite levels will soar in new tanks that have not yet been cycled. Nitrite is just as toxic to tropical fish as ammonia and the only way to quickly reduce nitrite levels is through a water change. Nitrites will eventually be converted to nitrate by the bacteria growing in the tank and filters. Ideally, in established tanks you want this reading to be 0 ppm with your aquarium test kit.

Nitrogen Cycle
This cycle usually takes from 2-8 weeks to complete and will happen in all new aquariums. You could speed up the process by using the filter material or gravel from an established tank. Even then it could still take a few weeks for the tank to cycle. This is the cycle whereby Ammonia is converted to Nitrites and Nitrites are converted to Nitrates. Please read the Aquarium Nitrogen Cycle article for more information.

Ammonia   ->   Nitrite   ->   Nitrate

pH is the scale used to measure the acidity or alkalinity of water. The scale ranges from 0 to 14 with 0 being the most acidic, 7 being neutral and 14 being the most alkaline. It is possible to raise or lower your pH levels with water changes or chemicals from your local pet store.

Phosphate can be introduced to your aquarium mainly from tap water, dead plants and fish food. High phosphate levels can cause algae outbreaks. There are products on the market to remove phosphates and you can do your part by keeping up with your aquarium maintenance and performing regular water changes. Saltwater reef tank keepers and freshwater plant keepers may want to invest in a phosphate test kit.

This is the amount of dissolved salts in water and is measured using a hydrometer.

Specific Gravity
This is a density measurement for the amount of dissolved salts in saltwater compared to freshwater. Explained another way, saltwater is composed of many more elements than freshwater. The specific gravity measurement shows us how much heavier or denser saltwater is compared to freshwater.

Water Hardness
The hardness level of water has to do with the amount of minerals that are dissolved in the water. Calcium and magnesium are the primary minerals that are dissolved in tap water. "Soft" water has relatively few dissolved minerals whereas "hard" water has many dissolved minerals. Water hardness is not really an issue unless your water is excessively soft. Then you may have problems with runaway pH levels. For saltwater aquariums this is especially true. The carbonate hardness of saltwater can give you a good indication of how stable your pH is.

Related Pages
pH : To be or not to be considered?
Cloudy Aquarium Water
Reverse Osmosis and Deionization

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Go here: Aquarium Water Forum