Keeping Fish

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Keeping fish

Your aquatic or pet shop will be able to provide you with expert advice on what proportion of water change is suitable for your tank or pond as many fish are sensitive to their environment.

Before you add tap water to your fish tank or pond, there are a few steps to take. This is because:

  • The temperature and acidity of tap water is likely to be different to the water in your tank or pond

  • Traces of metals are harmless to us but can be harmful to fish

  • The small amount of chlorine can be harmful to fish



What to do


Tap water is usually colder than your tank or pond. Fish are ectothermic, their body temperature is maintained by the water they live in.

Run the water and leave it to stand until the temperature rises to a level that matches your tank or pond before adding it.


Water that’s been standing in pipework can absorb tiny amounts of copper and zinc used in domestic plumbing fittings.

Run off your water from the cold kitchen tap (or the tap that’s fed directly off the mains pipe) for a while first to flush through the water that’s been sat in pipework. You can also buy special preparatory products from your local pet shop or aquarium to reduce any metals.


Tap water contains small amounts of chlorine to remove harmful bacteria.

  • Buy a specialist water conditioner or activated carbon granules from your pet shop to add to the water before using it.
  • Leave the water in an uncovered bucket for 24 hours allowing the chlorine to evaporate naturally
  • Use a fine spray when filling up your pond, which will aerate the water and remove chlorine.


Saltwater fish will have different pH needs to freshwater fish and sudden changes in acidity can be harmful.

Buy a pH test kit from your pet shop to test the water and adjust it where necessary.



This is a naturally occurring chemical that may be found in your tap water and be too high for your fish.

Your pet shop will be able to advise you further on how to measure and reduce phosphate levels.


This is a chemical compound in tap water. Most fish can tolerate higher levels of nitrate than the level in drinking water. Fish food, fish waste and fertilisers added around ponds can increase nitrate.  In certain conditions the nitrate degrades into smaller compounds like ammonia and nitrite.

To reduce nitrate degrading:

  • Remove decaying plants
  • Keep filters clean
  • Avoid over-stocking or over-feeding
  • Use low-nutrient compost if you have plants
  • Don’t use fertiliser in areas around your pond



If your tank or pond gets too much light, has high nitrate or phosphate levels or not enough plants, algae can start growing

  • Restrict the light by providing shade, keeping lights on for a shorter time and out of direct sunlight
  • Add fast-growing plants that will take up the extra nitrate and phosphates or introduce species of fish that graze on algae.

You can buy test kits to check chemicals and compounds in your tank or pond. However if you need more details than the kit can provide, you can arrange a free callback to request a water quality report for your area.