Whether you are the proud owner of a fish tank full of goldfish or you have something more exotic, like koi carp, swimming about in the garden pond, making sure that your aquatic pets stay in optimum health can be a delicate balance.   

For both regular and more tropical fish, maintaining a consistent level of water conditioning is crucial. The water’s relative hardness or softness can play a big part in that.  

The truth is that some fish prefer harder water, whilst others thrive on softened water. In their natural environment, certain types of fish thrive where there are higher mineral levels in the water, whereas others favour a less mineral-intense habitat. So, striking the right balance in your aquarium or ornamental pond is extremely important

As you may have read elsewhere in our Knowledge Centre, the hardness of the mains water supply in your home is determined by the amount of calcium and magnesium ions present in the water. Many people choose to remove the excessive hardness – which can be corrosive and even unhealthy – by installing a water softener. This will alter the level of carbonates and sulphates in the water, which is great for washing and cleaning and protecting your household appliances. At the same time, not everybody likes drinking softened water, so they have filtered hard water for their own consumption.  

With fish, different species respond differently to the levels of hardness and softness in water. It is essential that you only expose them to the water that is best suited to their long-term health and well-being. 

If you purchased your fish from an aquatics store, or a specialist fish farm, it is best to check with them first as to what type of water would be suitable for the species in question. 

In situations where you are considering installing a fish tank or a pond for the first time, it pays to do your research, as any old water may not do! If you have your eye on a particular type of fish, talk to experienced fish owners who have successfully kept them for many years. They’ll be able to explain what works and what doesn’t work.

What type of fish like hard water?

If you are using hard water in your aquarium, select fish that are accustomed to living in hard water conditions in the wild. Examples of these would be African rift lake cichlids, chromides, livebearers (like guppies, mollies and platies) certain types of rainbow fish and killifish, most freshwater gobies, brackish water fish and paradise fish.  

What types of fish prefer soft water?

One of the classic species that only ever does well in soft water is the lyre-finned checkerboard cichlid.  

Gouramis are also a smart choice for soft water fish tanks, along with most tetra, characins and rasboras.   

Unlike African lake cichlids that like hard water, many South American cichlids actually prefer soft water. So best check which continent they originate from! 

What’s best for goldfish and koi carp – hard or soft water? 

Well, goldfish live in freshwater and can handle both hard and soft water, although the way some water softeners soften water isn’t suitable for goldfish. Meanwhile, drinking water that has been filtered of its impurities through a reverse osmosis system is perfectly safe for goldfish. 

As for koi carp, considered opinion varies. Some say hard water is best for them, since soft water can affect their bodily processes, whilst others reckon soft water is preferable. 

Quick water tips for fish keepers 

Always check whether you should dechlorinate your tap water before it goes into the aquarium and make sure you compare your water hardness to the one at the aquarium where you purchased the fish, to ensure that they aren’t going to re-homed in an alien environment. Thereafter:  

  1. Routinely measure the levels of water hardness
  2. Ensure the fish tank filter is always kept clean and in good condition, to maintain a consistent water quality level  
  3. Monitor and control the water temperature, pH and ammonia levels, the amount of dissolved salts, minerals and nitrates.  

Please note, that at Scott Jenkins Water Softeners, we supply water test kits to help you measure the different components in your water.

An aquarium can look spectacular in the home environment but before populating your tank with fish, we always advise our customers to check with their stockist or a fish expert, to make absolutely certain the water hardness or softness is going to be appropriate.    

Scott Jenkins Water Softeners is one of the leading water softener companies in the south of England, with particular focus on West and East Sussex, Hampshire and Surrey. We look forward to being of service.  Tel 01243 607494 or email: scott@sjbs.info 

Head here to get an instant water softener quote.