Aquarium pH & Hardness

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PH and hardness are important factors to consider when keeping demanding tropical fish, or if you live in a particularly hard or soft water area. Getting an idea of how pH and hardness works will help to ensure the best health of your fish.

Dip strip aquarium hardness test

When does pH and hardness need altering?

For most general community aquariums, pH and hardness are not major causes for concern. If you live in an area with higher or lower than average conditions, your local retailer(s) will usually acclimate the fish they sell to local conditions.

When a fish species demands different conditions, a retailer should keep these fish on a separate system clearly marked as being hard or soft water. Of course, you should always look up a fish or have a general idea of its requirements before you purchase anyway.

The need to alter pH and hardness comes when we keep fish which are outside of the range of our water supply, for instance keeping soft water fish in a hard water area or vice versa. Water conditions may also need to be altered if any breeding is planned, since although many fish will live perfectly happily outside of their natural range of conditions, they are far less likely to breed.

The difference between pH and hardness

Although pH and hardness are different properties of water, they are closely linked. Whilst pH is a measure of the acidity and alkalinity of water, hardness is a measure of the dissolved minerals in the water. The two are closely linked because dissolved minerals tend to counter the effects of acids in the water (a process known as buffering), preventing the pH from dropping. In most cases therefore, hard water usually has a high (alkaline) pH, whilst soft water has a low (acidic) pH.

As with all things however, there are exceptions, and it is possible to have differing levels. In my area, where the water is hard, I see many aquariums which have been running for a few years without adequate water changes which have hard water but a low pH. This occurs because the acids produced in an aquarium (from fish waste, respiration, bog-wood etc) have 'overtaken' the buffering capacity of the elements in the water, causing the pH to drop, but leaving sufficient 'non-buffering' minerals for the water to remain hard.

For the vast majority of fish species, if they prefer hard water, they will also prefer a neutral to high pH, and soft water fish will prefer a neutral to low pH.

Raising hardness and pH

Making water harder is very easy since we are adding minerals to the water rather than taking anything out. The best method of creating hard water is to use off the shelf mineral and trace element additives. These powdered mixes of minerals can simply be added to any water being prepared for the aquarium. Follow the instructions and monitor your conditions to ensure you are getting the desired levels.

While mineral additives give the ideal conditions for hard water fish, they can be steadily diminished by acid producing processes in the aquarium. To keep the water at a steady hardness and high pH you can use rocks and substrates designed for marine aquariums such as tufa rock, ocean rock, or coral gravel. All of these will raise and/or maintain hardness and pH levels over very long periods, giving a stable environment for your hard water fish.

Reducing hardness and pH

Lowering pH and reducing dissolved minerals (hardness) is a little trickier and there is more scope for things to go wrong. There are two basic options for reducing hardness; attempt to alter the water parameters or use a different source of water. The latter of these options often turns out to be the easiest.

You can remove dissolved minerals and reduce pH by adding acidic (pH reducing) treatments to the aquarium, although these are best used when the water is not overly hard or alkaline and just needs a gentle 'nudge'. Water softening 'pillows' or peat-based medias can also be used in external filters to help maintain soft and acidic water.

Treating water at source (i.e. before it reaches the tank) is safer and can be done with ion-exchange filters attached to a mains supply. These filters are cartridge based and will need to be either replaced or recharged by running salt water through the unit on a regular basis. If your water is particularly hard, it can become costly to continually replace these filters and using a new source is often far easier.

A better source of water for your tropical fish

An alternative to tap water is to use reverse osmosis (R.O.) water. R.O. water is pure water with no minerals or pollutants and can be obtained from most retailers for around £3-4 for 25 litres. You can also produce your own R.O. water by purchasing a reverse osmosis unit, which can be easily plumbed into your mains supply. Expect to pay around the hundred pound mark for a unit, but this can be easily recouped if you use a reasonable amount of water.

As a rough estimate, taking into account cost of water and replacement membranes etc. 25 litres of R.O. produced at home would cost just under a pound. If your shop charges £3 for 25 litres, you would get back your £100 for a unit after producing 1250 litres (50 x 25 litres) This is a very roughly estimated figure however, since there are too many factors to consider, so it could turn out cheaper or more expensive.

Because the membrane in these filters can wear out quickly if it is continually turned on and off, you will need to produce and store a few hundred litres at a time, meaning space and storage should be a consideration, and you will also need a waste drain since the units produce more waste water than filtered water.

Once you have your pure, R.O. filtered water, you will need to add some minerals back; pure water has no buffering and could result in dangerously wild fluctuations in pH. This is easy to do and most retailers sell suitable additives and buffers depending on your desired levels. The end result of R.O. water with added minerals is just about as good as you can get for your fish, although close monitoring is still essential to ensure some level of hardness is always present.

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