Of all the things that kill tropical fish, water conditions have to come first on the list, which is closely followed by overfeeding and then environmental control. The Feeding part will be covered in the relevant section, as will the Environmental Control which will cover the heating and living requirements of your fish. Diseases kill fish also, but we will learn over time that a very high percentage of diseases are cause by incorrect water conditions, so by maintaining good water standards we can reduce most of these diseases to a minimum.

Here we will be talking about the things in the water that can stress, and even kill your fish very quickly, and for this particular article we will concentrate on the Ammonia, Nitrites, Nitrates, and pH, with a mention of hardness, the dH.
In this age of Bottled Water we have learned that even our own tap water is not safe to drink in many places, and in many instances is also unsafe for your fish unless we take precautions to bring it to the right condition that is suitable, and then learn how to keep it there.

As soon as you place fish into your tank certain things begin to happen, mainly that the wastes from your fish in the form of ammonia begin to mix with the water. This ammonia is toxic and if allowed to build to high levels can kill your fish, so we have to control it to a safe level that the fish can tolerate. This is commonly known as the Nitrogen cycle.
The waste products produced by the fish undergo a process where certain bacteria go to work and begin to break the wastes down into minerals. These bacteria occur naturally, but can only cope with so much waste. In a heavily planted tank much of this waste will go to providing the plants with food, but again, there is a limit to this also.
Filtration will help with this process, but is a separate topic, so we will concentrate on the water as such.

As the ammonia levels rise in the aquarium, another group of bacteria feed on the ammonia and convert it to nitrite (also toxic above certain concentrations). Nitrite is consumed by more bacteria and is converted to nitrate, which is relatively beneficial at low levels. Nitrate levels however will continue to rise in the aquarium unless removed through partial water changes and filtration or utilized by live plants.

Ammonia

As we mentioned earlier, the majority of fish wastes produced in an aquarium is ammonia or converted to ammonia.
Ammonia exists in water in two forms: as toxic molecular ammonia(NH3), and as the nontoxic ammonium ion(NH4+).
We need to avoid getting too technical here but the amount of ammonia in your tank can vary with the level of pH, for as the pH increases, so does the toxic ammonia level increase. An increase in temperature combined with a rise in pH can make this toxic level even higher.
Ammonia, which destroys the mucus membranes of fish, is secreted by fish directly through the gills as a waste product, and by the bacterial breakdown of fish waste, uneaten food and plant by-products. This causes the ammonia levels in the tank to reach toxic levels, known as ammonia poisoning.
If you intend to increase the pH level of your water for some reason such as breeding a particular species, you should first check the ammonia level in your tank before doing so, as any change in pH level could give disastrous results. Should the ammonia be at a high level you may have to consider several water changes before continuing, but do this over a period of a few days, not all at once.
Many of the ammonia test kits give explicit instructions on testing ammonia levels, and these should be followed to the letter.
You will need an accurate reading of your pH and temperature in order to work out the levels of ammonia in your tank, but having done it once you will find it easier the next time.
Any level above .05 parts per million is considered not good and could lead to either great stress or even death to your fish, so this is something you need to become familiar with.

Signs of increased toxic ammonia in your tank are fish gasping at the surface, inflamed gills on fish, cloudy eyes or frayed fins, but any unfamiliar behavior by your fish should lead you to checking the varying conditions of your tank. Think of ammonia as a weak acid solution that is breaking down the natural covering of the fish. All fish have a slightly slimy coat which is a natural defense against disease. Once this is weakened your fish become sick, open to attacks of disease, or in the worst cases they die.
Although salt is associated with marine aquariums, freshwater aquariums can benefit from small doses, as well. A lack of sodium (salt) in the water will break down the slime coat of fish. One tablespoon per five gallons will provide enough salt for most fish. Mollies, Swordtails, Tetras, Fancy Guppies, and Goldfish prefer a little more. Cichlids, Knifefish, and Ghosts prefer less. Only use salt that is recommended for aquariums. Never use regular table salt which may contain iodine.

Water Changes

Water changes are the best way to immediately solve ammonia problems. If you do partial frequent water changes, over a period of days, this will help to reduce the ammonia levels. Also try to bring the pH level to as near to neutral (7.0) as possible because this in turn will reduce the amount of toxic ammonia in the tank and change much of it into nontoxic ammonia.
Avoid using chemicals if possible, as this is the quickest way to upset the balance of your tank. Use of chemicals can also kill the beneficial bacteria that you have established in the tank.

So What Is Nitrite?

Nitrites are by product of the ammonia, but just think of it as the bad bacteria in your water, as it is easier to remember this way. Nitrite at high levels is toxic to fish because it prevents the fish's ability to use oxygen.
Nitrites are converted by beneficial bacteria into nitrates. High levels of nitrites indicate that the breakdown of organic materials isn't complete and that the aquarium does not have adequate biological filtration. Nitrites are less toxic than ammonia, but can still kill fish if the levels are too high. Nitrates, on the other hand, are generally harmless to freshwater fish at low levels and are actually used by plants and algae as food. Test kits are available that test the levels of both nitrites and nitrates.

Nitrite is also pH dependent. If nitrite is present in the aquarium and the pH value falls below 6.5, the nitrite will be changed to nitrous acid, which is very toxic to fish. Levels above 1.0 ppm for tropical fish of the freshwater variety are considered unsafe.
As with ammonia, the quickest way to remove nitrites is with partial water changes. In this case daily water changes must take place over say a week and this will quickly dilute the nitrites in the water and bring them to a safe level.
If you can possibly move the fish to another tank that you know has good established water then the chances of saving your fish from poisoning will be greatly increased. Don't be tempted to put your fish back into the tank you removed them from until you are absolutely certain that the ammonia and nitrites are at a safe level. You should have no nitrites at all before returning the fish. Once you have reduced the levels of ammonia and nitrites, just add a small number of fish to keep the cycle working. Never stop your filter for prolonged periods, as this will reduce, or even kill off the good bacteria it holds.

If you get into the habit of SIPHONING the bottom of your tank instead of VACUUMING, then much of the water that you remove can be replaced by clean water that you hopefully have standing by for top ups. Don't return the water you siphoned off as this will defeat the purpose. The top up water should preferably be aged water and should be at the right temperature BEFORE adding it to the tank.
Many people trickle feed water into the tank direct from the tap, but this should be avoided if possible. Find a container that will hold ten gallons or so and keep this for your top ups, or better still have a spare aquarium full of good water that you can use. Even a covered tub in the back yard can serve this purpose, but you must warm it before you add it to your tank.

Nitrate Levels.

As we said above, Nitrites are converted by beneficial bacteria into nitrates. High levels of nitrites indicate that the breakdown of organic materials isn't complete and that the aquarium does not have adequate biological filtration. Nitrites are less toxic than ammonia, but can still kill fish if the levels are too high. Nitrates, on the other hand, are generally harmless to freshwater fish and are actually used by plants and algae as food. Test kits are available that test the levels of both nitrites and nitrates.

Nitrate (NO3-) is the end production of the nitrogen cycle and is relatively nontoxic. Plants and algae use nitrate as a food source, which will reduce the levels. They say that plants should not be added to the new aquarium until nitrates are present, but personally I like to add a few at the very beginning of setting a tank up. If the plants are from an established tank so much the better to get things moving, and even a few hands full of gravel from an established tank will help get the bacteria moving, and this along with filtration will set you off to a good start. Again, a filter from an established disease free tank will help speed things along because of the beneficial bacteria it holds.
Provided no fish are present in the tank then all should be well.
Many aquarist combat cloudy water, (which is usually a sign of increased bacteria presence), by adding a few hands full of gravel from an established tank. The good bacteria in this gravel go to work quickly and combat the bad bacteria and therefor the cloudiness goes within a few hours.
In freshwater aquariums, the appearance of nitrates suggests that the biological filtration system is operating. It is recommended that nitrate levels be kept under 60 ppm. A partial water change is the most effective way to reduce nitrates.

Water Hardness

Soft water has around zero to 75 parts per million, expressed as (ppm)
In the middle, between hard and soft is the moderate area, which is around 75 to 150ppm.
Hard water is between 150 and 300ppm.
Soft water usually has a low pH, in other words below neutral (7.0), whereas hard water usually is above neutral.
Different fish require different conditions, but as our pH range in general falls between 6.5 and 7.8 at the outsides we would therefore have a hardness in the region between a few parts per million and 150.
Generally you would stay within the middle range as above.
The hardness of water in aquariums is usually arrived at by adding a few drops of a special liquid soap to a measured amount of aquarium water in a small container. The amount of drops added to make the water lather gives a reading of the hardness of the water.

pH (Power of Hydrogen)

pH is a measure of the amount of hydrogen ions in your water.

The pH scale runs from 0.0 to 14.0. Values less then 7.0 are Acidic, 7.0 is Neutral, and values greater than 7.0 are Alkaline.
Alkalinity is a measure of the buffering capacity of water. Because pH is a logarithmic function, a change in pH from 6.0 to 7.0 would represent a ten-fold decrease in the hydrogen ion concentration; 6.0 to 8.0 would be a 100-fold decrease. In other terms a pH of 6.0 is 100 times more acidic than a pH of 8.0.
Properly buffered water has a 120 to 240 parts per million (ppm) total alkalinity reading.
For most species of fish, the pH in a freshwater aquarium should be between 6.5 and 7.8. At pH levels below 6.5, the growth and survival of nitrifying bacteria becomes reduced and could possibly cease altogether. If pH levels below 6.5 are to be used like for breeding a certain type of fish, then frequent water changes are vital to prevent the accumulation of nitrogenous wastes (ammonia).

pH in the aquarium is an important factor. Not only do fish require a certain pH range to survive, but pH is an important controlling factor for many chemical balances, including the ratio of nontoxic ammonium ion (NH4 +) to toxic ammonia (NH3), and between the toxic nitrite ion (NO2 ¯ ) and very toxic nitrous acid (HNO2).

You can in most cases correct the pH level without the use of chemicals.

Water changes will replace some of the natural buffers in your aquarium. These water changes should be small, perhaps 25% or less, as removing too much water all at once will once again upset the balance of the tank. Again, used aged water if possible and make sure the temperature is consistent to the water in the tank.

Raising the pH.

To make the water more Alkaline (Raise the pH) use something like Dolomite limestone or oyster shell in your filter, or even a bit of crushed coral, but anything you add should be changed often as the values of these will become ineffective over time. You should check the water condition every day when altering the pH and this must be done gradually over time to avoid shock.

Lowering the pH.

Again, avoid chemicals if possible. You can use filtered rainwater to bring the pH down (Below 7.0) Remove 25% of the water and top up with rainwater that has been warmed to the right temperature. Test the water, and then again in an hour or so when things have settled down.
You may have to do this several times, but not all at once, just do it gradually over a few days. Storing water in a warm place will help you in this task, and in summer it might be possible to store water outside in a container covered in black plastic. This may generate enough heat to keep the water at an acceptable level. A hot water cupboard might help in the winter months, as long as you are careful for obvious reasons.

If you place a layer of peat moss in your filter this will also gradually make the water slightly acid and soften it slightly at the same time. The water may take on a faint yellow colour or even turn brown, but this can be removed with activated carbon in the filter.

Whichever method you use, do it slowly over a few days to avoid shock and stress to your fish.

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