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Tropical Fish Tank

It all starts with fresh water

Like us humans that need good air to live, fish need good water to survive in a tropical fish tank.

And there are two aspects to this; the water chemistry and the water quality.

Water chemistry is all about the amount of organic and inorganic substances in the water that affect the acidity or the alkalinity of the water

Water quality is all about the amount of undesirable, mainly organic, material in the water.

Water Chemistry

There are three main elements (excuse the pun) to the chemistry of tropical freshwater aquarium fish keeping; hardness, pH level and salinity.

Now that’s not to say that all fish need the same water. And so when I select the fish for my tropical fish tank I need to make sure they can tolerate similar levels of hardness, pH level and salinity.

Water Hardness

I can tell if I live in a hard water area or a soft water area simply by the amount of lather in my bath water.

The level of hardness is mostly affected by the amount of calcium dissolved in the water and is measured by the number of parts per million of calcium carbonate in the water.

One important point to remember is that a water sample can be measured as soft, that is containing a small amount of calcium carbonate, yet still be rich in other minerals. And soft water fish that come from waters that are soft and with small amounts of other minerals will not fare well in artificially modified water because the amount of calcium carbonate might be corrected but other minerals in the water remain.

Fish originating from hard water areas may adapt to a softer water environment but fish originating from soft water areas may not be able to tolerate a harder water environment.

Water hardness is measured by a process called titration. This involves adding a small amount of special solution to a water sample until the sample changes colour.
Test strips are also available. A sample of water is put on the strip and the test strip changes colour. The colour is matched against a chart to work out the hardness of the water sample.

pH level

The pH scale measures the acidity to alkalinity of any substance.
The pH level is measured on a scale from 0 – 14 where 0 is is dangerously acidic, 14 is absolutely alkaline, with 7 being neutral.

The scale is logarithmic. The curve is not a straight line. That is, a substance with a pH value of 8.0 is ten times more alkaline than a substance with pH value of 7.0 and one hundred times more alkaline than a substance with pH value of 6.0

The pH of water is affected by its mineral content and by the amount of carbon dioxide and organic materials in the water.

The majority of fish can live in free water with a pH level ranging from 4.0 to 9.0 and a maximum annual variation of 0.5. So this is the environment we need to produce in our aquariums.

And it goes without saying that when selecting my fish I need to consider fish with similar environmental requirements.

The pH level of water can be measured with a pH meter, a field kit or a basic litmus paper test.


How much salt is in the water and the most common element is common salt, sodium chloride.

Measured in parts per thousand. Tropical seas having a typical saline level of 35 gm of salt to 1 Kg of water.

More commonly measured by taking a reading of the specific gravity using a hydrometer.

However the SG of a liquid varies with temperature. Typically a saline solution with 35 ppt has an SG of 1.026 at 15 DegC and an SG of 1.024 at 25 DegC

And then there’s brackish water a halfway house between marine and freshwater solutions. With a typical salinity of 15 ppt.

But that’s only really a concern for marine aquarists. For now lets focus on freshwater aquariums, yes?

How to Adjust the Water Chemistry

I’ll be pretty lucky if I can take water straight from the tap and find it suitable for fish to survive and flourish.

Making the Water Harder

To make water harder add calcareous decor materials like coral sand. There are also salts available that can make the water similar to that of Lake Tanganyika.

Making the Water Softer

To make the water softer takes a little more effort.

Water Softening Resins.

Hard water is passed through the resin and comes out a lot softer. However the chemical process converts the calcium ions to sodium ions. So although the water coming out is softer it is also still rich in minerals and unsuitable for most soft water fish species.

Rain Water

Rain water is usually depleted of minerals and neutral or slightly acidic. Unfortunately it does collect contaminates on its way down so to make sure it should be strained through activated carbon to remove the residual contaminates before further treatment.

Pure Water

The safest and most effective method of providing soft water is to use distilled, de-ionised water or pass the water through a reverse osmosis machine.
All mineral salts and organic contaminates are removed.
But so is all the free oxygen needed for the fish to breath. So the water needs to be aerated before introducing any fish.

Adjusting the pH Level

Increasing the pH level can be achieved by adding calcareous material.
For a quick fix add a small dose of bicarbonate of Soda. But check the hP levels between doses.

Decreasing the pH Level

Making the water more acidic is best achieved by using a filter containing peat.
Only use pure peat. Horticultural moss peat without any fertiliser additives can be used or buy a proprietary product.

There are also chemical pH adjusters which should be used with care because the effect is dramatic and instantaneous.

Buffering the pH Level

Fish waste and plant byproducts are slightly acidic and will over time reduce the pH level in the aquarium. Counter this by including some calcareous decor. As acids are produced the pH level reduces. The acids dissolve the calcium carbonate which makes the pH level rise again.

Other Water Parameters

There are also two more chemical related parameters that need to be considered: Oxygen content and temperature.

Oxygen Content

Fish need oxygen to breath.
In the wild river water is oxygenated by the flow of the water over rocks and waterfalls. In lakes by the waves and surf increasing gas exchange at the surface – carbon dioxide from the water replaced by free oxygen from the atmosphere.

Adequate oxygen in aquarium water is ensured by providing a reasonable amount of water circulation.
This can be achieved by passing a steam of air bubbles through the water. This increases water circulation bringing the oxygen depleted water at the bottom of the tank to the surface to be re-oxygenated at the water-air interface .
Movement of the water surface will increase the carbon dioxide – oxygen exchange and is achieved by directing the outlet of the filter towards the surface of the water.


Fish are very susceptible to dramatic changes in temperature. Most fish cannot generate body heat and must adapt to the temperature around it. When the temperature falls the fish are sluggish. When the temperature rises their metabolic rate increases with a corresponding increase in appetite and rate of breathing. As water temperature increases the amount of oxygen decreases making it harder for the fish to breath.

Water Quality

The quality of water is affected by organic and inorganic contaminates when drawn from the tap and also contaminates introduced when the water is in the aquarium.

Inorganic Contaminates

Tap water contains small quantities of various metallic salts either naturally or through environmental pollution. Also it may be contaminated by passing through old lead or copper water pipes. Water supply companies also add chemicals like chlorine and chloramine to the water supply to kill bacteria. Now while the levels of contaminates in the public water supply may be fit for human consumption, they may not be safe for my aquarium fish.

How to Purify Tap Water

Chlorine can be removed by allowing the water to stand over night or running the tap water under pressure into a bucket.

Chloramines and other nasty toxins need to be removed by adding a water conditioner.

Improve the water quality even more by using a tap water treatment filter which will remove most nitrates, phosphates and sulphates.

Inorganic contaminates can also be introduced into the aquarium by introducing unsuitable decor or accidentally adding insecticide spray, furniture spray, paint fumes and the like.

Organic Contamination

Organic contaminates may be already in the tap water used to fill my aquarium.
The tank occupants, fish and plants, are the main cause of organic pollution inside the aquarium.

This is controlled naturally by the nitrogen cycle.

The Nitrogen Cycle

The nitrogen cycle is natures biological way of dealing with waste products.

The cycle starts with the decomposition of dead plants and animals and the excreta of animals. The decomposition breaks down the material into highly toxic ammonia and associated compounds.
A bacteria called Nitrosomonas eat this ammonia which converts the ammonia into a less toxic nitrite.
Another bacteria called Nitrobacter eats the nitrite and converts it into nitrate.

Nitrate is not harmful to fish and is a natural plant fertiliser.

Nitrosomonas and nitrobacter bacteria are aerobic, they need oxygen to survive and the bacteria inhabit any suitable surface like wood, sand and plant leaves.

In nature the nitrogen cycle is continuous and creates a balance. However in a confined space the nitrogen cycle processes need a little help. And we help out nature by biologically filtering the water in the aquarium.

Another reason to filter the water is to collect all the solid waste and uneaten food which if left in the tank would cause some serious pollution on its sown.


Aquarium filtration systems operate mechanically to remove solid waste, biologically to remove the toxic ammonia and chemically to remove other unsavoury toxins.

The basic principle is that the tropical fish tank water is passed through a material that traps the solid waste.
This filter material is usually plastic foam or gravel. But on its own this mechanical filter will get clogged up very quickly and will need frequent cleaning.

So I need to allow the mechanical filter to grow its own biological filter.

Biological filtration

Biological filtration uses the nitrogen cycle bacteria to treat the waste collected by the mechanical filter.
This bacteria needs a surface to cling to, to live and to multiply.
Suitable filter media for biological filters include sand, gravel, expanded clay and specially designed plastic shapes that have large surface areas.
This media acts as a mechanical filter to begin with but as the nitrogen cycle bacteria increases the biological filter effect kicks in.

So, take note, a newly installed biological filter needs time to get established. Do not introduce fish into the tropical fish tank until the filter system has matured.

Chemical Filtration

Chemical filters work best in a high flow of water and also act as a mechanical filter requiring periodic cleaning.
The chemical media include activated carbon and zeolite. Chemical filter media are also available to remove nitrates and phosphates.
Chemical filter media have a useful life and will need replacement after some time.

Well that’s only the beginning of our journey into keeping freshwater fish in  a tropical fish tank. Next up we focus on the various filters available…


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