A Guide To Fish Tank Chemistry

Getting your fish tanks water quality right is the key to ensuring that you have a healthy thriving aquatic environment. Because fish are very sensitive you need to be very careful with any changes you make to your tank. Poor water quality leads to stressed disease prone fish.

This article sets out some the basic principles behind water quality and treatment.

Ph (Acidity and Alkanicity)

Ph measures how acidic or basic your water is. Any changes to the ph level should be done slowly. You can test the ph of your tank using a ph testing kit such as . The PH scale runs from 0 to 14 with 0 being the most acidic and 14 the most alkaline. Most fish prefer a ph between 5.5 and 9.0. Because the scale is logarthimic each step up or down the scale represents a 10 fold increase or decrease. This shows how small changes in the ph represent big changes in your tank environment.


Nitrites are produced as part of the natural decomposition process of your tank’s constituents. Fish waste, uneaten food and plants are broken down by micro organisms. Ammonia is degraded to nitrite and then nitrate. Nitrates should measure between 0 – .3 mg/l. Your tank should not exceed .8 mg/l so you need to test your tank regularly.
If you have a persistent problem with high levels of nitrates the most likely cause is a dirty filter or excess waste in the tank.


Ammonia is very harmful to your fish. Where you Ph measures above 7 toxic ammonia will increase. Concentrations higher than .25 mg/l are extremely toxic to your fish.
Changing your gravel and partially changing your water will help to bring levels down. Clean your filtration equipment and make sure that your water is well aerated, oxygen helps break down ammonia.


Nitrates are produced as a result of the natural process of breaking down organic wastes. While less toxic than nitrite and ammonia, the other two memebers of the nitrogen cycle for fish tanks, they cause an increase in algae growth and fish stress.


Salinity is the measure of the salt content in fish tank. Aquarium water is made of tap water and marine salts. Marine salts contain trace elements that are good for your fish. Never use domestic salt in your fish tank!


Phosphate levels should be monitored weekly. High levels are a result of overfeeding or poor water quality and results in increased algal growth. Removing dead plants and excess food should decrease your phosphate levels. Tap water may also contain high levels of phosphates so be sure to test it before adding it to your aquarium. In saltwater and freshwater fish tanks the ideal level of phosphates is from 0 – 1 mg/l

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