Water for Aquariums
Understanding and maintaining proper water quality for healthy fish and aquatic life
Water Quality Fundamentals
Water is the fishes atmosphere and serious consideration must be given to its supply and condition. The fish we keep in our aquarium can come from several different types of water depending on its origin. It is not too difficult to maintain a successful aquarium with a collection of fish from different continents. However some fish are fussier in their requirements and may die if given the wrong water conditions. If you plan to breed your fish then a much deeper knowledge of chemistry is needed. There are still many fish that have not been bred in captivity; this is due to us humans not being able to mimic the water conditions of their natural environment.
Important: Chlorine and Chloramine are added to water to eliminate harmful bacteria in drinking water for human consumption, but are very toxic to fish, beneficial bacteria and plants. When first setting up an aquarium and whenever partial water changes are conducted, always use water conditioner to make the tap water safe for fish.
It is important to ensure that you maintain the correct water conditions for you fish. This can be monitored weekly by use readily available water testing kits. Most water chemistry can be altered by the vast array of products available today.
- pH Level
- General Hardness (GH)
- Carbonate Hardness (KH)
- Ammonia Levels
- Nitrite Levels
- Nitrate Levels
- Chlorine/Chloramine
- Water Conditioners
- Reverse Osmosis
- pH Adjusters
- Water Testing Kits
- Beneficial Bacteria
- Regular Water Changes
Water Treatment
Reverse Osmosis
Reverse Osmosis filtering of water is done by forcing the water under high pressure through a porous membrane. The holes in the membrane are large enough for the water to get through but trap larger particles which would be impurities. The "high pressure" part is the key as the holes in the membrane are so small that nothing would get through without it.
Acidity / Alkalinity
Acidity and alkalinity are measured on a scale of 1 (very acid) to 14 (very alkaline). Both extremes are lethal to fish and plants, especially if you bear in mind that water having an acidity of 6 if tem times as acid as water with an value of 7. Acidity/alkalinity is represented by the symbol pH, with pH7 indicating neutral conditions. All values below pH7 are increasing levels of acidity, while those above are increasing levels of alkalinity. Most freshwater fish prefer pH values between 6 and 7.5.
Hardness
Hardness is also important when creating a 'natural' environment for your fish. Hardness is due to dissolved salts from calcium and magnesium. There are 2 types of hardness general (GH) and temporary (KH). When temporary hardness is low, water loses much of its buffering capacity and can lead to rapid fluctuations in quality. This can be removed by boiling your water. General hardness is important in a different way. Fish that originate in soft water e.g. the Amazon need low GH, while fish from African Rift lakes need a higher GH.
Ammonia
Ammonia is released by most living organisms as part of the break down of organic matter. Fish waste contains a high level of ammonia. Ammonia is very damaging to fish health, it irritates the gill and skin surfaces, destroys the protective mucous coating of the fish. Even at levels below 0.01 ppm ammonia has been shown to lower the immune system of fish.
Nitrites / Nitrates
During the natural biological purification, ammonia is converted into nitrites that are also toxic to fish. Nitrites must be closely monitored in new tanks set-ups. However with a good population on filter bacteria these nitrites are converted to nitrates, whose concentration needs to rise above 50mg before becoming toxic to delicate fish species. Nitrates can be kept at a minimum by regular water changes.
Tap water treated with a water conditioner is generally best for most aquariums. Softened, dechlorinated, and temperature-stabilized water provides a safe environment for fish.
Yes, tap water is fine if treated with a water conditioner to remove chlorine, chloramines, and heavy metals before adding fish.
Neither is ideal alone. Distilled water lacks minerals, which fish need, while spring water may have high or inconsistent mineral content. Tap water treated with conditioner is usually best.
If using a water conditioner, tap water can be used immediately. Without conditioner, tap water should sit 24–48 hours to allow chlorine to dissipate.
Look for fish breathing normally and active, bubbles at the surface, and use an oxygen test kit. Plants and aeration devices help maintain oxygen levels.
Yes, but it may need mineral adjustment. Bottled water often lacks essential minerals for fish health. Use it cautiously and test pH and hardness.
Untreated tap water can be harmful due to chlorine or chloramine. Fish survival time varies, but exposure even for a few hours can be fatal without proper treatment.
Fish prefer water that matches their natural habitat. Freshwater tropical fish prefer slightly soft, neutral to slightly acidic water, while other species may require harder or more alkaline water.
Use a water conditioner to neutralize chlorine and chloramines. Optionally, remove heavy metals and adjust hardness or pH depending on fish species.
Use spring or mineral water with low to moderate hardness and neutral pH. Avoid distilled or purified water unless minerals are added for fish health.
Let tap water sit for 24–48 hours to allow chlorine to evaporate. This does not remove chloramines, so using a conditioner is safer and more reliable.
Yes, Brita-filtered water removes some chemicals and metals but may not remove chloramines or fully condition water. Always check pH and hardness before adding fish.
Mix tap water with reverse osmosis water or use commercial softening products. Test water hardness and pH to match fish requirements.
Yes, most water conditioners are safe to add directly to the tank even with fish present. Always follow dosage instructions on the product.
Change 10–25% of the water weekly, use water at the same temperature, and treat tap water with conditioner before adding to prevent stress or shock.
Use treated tap water, check temperature, pH, and hardness. Let it sit for a few hours after conditioning and test parameters before adding fish.
Partial water changes of 10–25% once a week are recommended to maintain water quality and reduce nitrate build-up.
Maintain proper filtration, perform regular partial water changes, use treated water, and monitor pH, hardness, ammonia, nitrite, and nitrate levels.
If done slowly with temperature-matched, conditioned water, partial water changes do not significantly stress fish.
Use a water conditioner to remove chlorine, chloramines, and heavy metals. Optionally adjust pH and hardness for the species you keep.
Use an aquarium nitrate test kit. High nitrates indicate a need for partial water changes or better filtration.
Ammonia should always be 0 ppm. Any detectable ammonia is toxic and needs to be addressed immediately through water changes or filtration.
Nitrites form from the breakdown of fish waste by bacteria in the nitrogen cycle. High nitrites indicate incomplete cycling or insufficient filtration.
Most freshwater tropical fish can live in treated tap water. Always condition tap water and check pH/hardness to match species requirements.
Yes, tap water can be used for top-ups if treated with water conditioner to remove chlorine and chloramines.
Untreated tap water may contain chlorine, chloramines, or heavy metals that are toxic to fish. Always condition water before adding fish.
Yes, but only after treatment with a water conditioner to remove chlorine and chloramines. Test pH and hardness if keeping sensitive species.
Yes, always use a water conditioner to remove chlorine, chloramines, and heavy metals before adding tap water to an aquarium.
Fish cannot survive more than a few minutes out of water. They require water for oxygen exchange; any exposure beyond a few minutes is fatal.
Pure distilled water lacks essential minerals, so it should be mixed with conditioned tap water or remineralized before use.
Treated tap water is usually best. Condition it to remove chlorine/chloramines and adjust hardness or pH as needed for your fish species.
Filtering tap water and using a water conditioner is usually more cost-effective and consistent than relying on bottled water.
Use proper filtration, perform regular partial water changes, avoid overfeeding, and maintain a balanced ecosystem with plants and beneficial bacteria.
With a water conditioner, tap water is safe immediately. Without conditioner, let it sit for 24–48 hours to remove chlorine.
Yes, but check pH and hardness. Bottled water may need mineral adjustment to support healthy fish.
Most conditioners work immediately, so fish can be added right away. Always follow product instructions for best results.
The most common cause is poor water quality, including untreated tap water, ammonia, nitrites, or extreme pH changes.
Neither is ideal alone. Treated tap water is preferred. If using spring or distilled water, adjust minerals and pH to meet fish needs.
Use a water conditioner to neutralize chlorine and chloramines. Letting water sit only removes chlorine, not chloramines.
Beneficial bacteria in the filter and detritus-eating organisms like snails or shrimps help break down waste naturally.
Full water changes are rarely necessary. Partial weekly changes of 10–25% are safer and maintain stability for fish.