Aquariums use a lot of water, so it sensible to want to reuse this where we can. But is aquarium water safe for watering plants? Neale Monks advises.
I’d like to use my old aquarium water for the garden in dryer months, especially as the fish waste will be beneficial to the plants. However, I’ve been using tonic salt in the water, at between 1-3g per litre, to help keep my fish healthy. Will the salt in the water damage the soil or plants in my garden over time?
HELEN CHESSHIRE
Neale advises: In theory, aquarium water is great for the garden – it’s full of nitrate and phosphate, and all the decomposing bits of leaves and whatnot adds to the organic content of your soil. It's also good for houseplants as well. So, there's no real reason not to use aquarium water in the garden. Unless, that is, you add salt.
Whether or not a low dose like yours would cause harm is difficult to say for sure because it'll depend on a lot of factors. A few litres of water at the salinity you're using could well be diluted into oblivion with the next heavy shower of rain. But then again, if it doesn't rain for a while, or your soil has poor drainage, or your plants are sensitive to saline or alkaline conditions, even a low salt level could cause problems.
Now, the deeper question is why you're adding tonic salt at all. It's almost always unnecessary, and the use of tonic salt is more of a hangover from fishkeeping in the 60s and 70s than something recommended today. Traditionally, it had maybe three uses in the hobby. One was a medication, where the addition of a little salt (1-2g/l) could be used to treat whitespot. Dosed at this level, for a week or two, the salinity produced was usually enough to end the cycle of infection without the need to use other sorts of medicine. The second was when people were keeping hard water fish, like guppies, in areas with soft water. Adding salt allowed the guppies to osmoregulate better, reducing the risk of disease. Finally, there was the idea that salt reduced the toxicity of nitrate, making mild doses of salt useful when nitrate-sensitive fish, such as mollies, were being kept.
Tonic salts were used to help fish like guppies osmoregulate better, Photo by Shutterstock.
But the routine addition of salt doesn't add anything to a freshwater aquarium that can't be managed better in other ways. Hard water fish are better kept in water that's hardened using something like Rift Valley salt mix, and in fact the use of salt as an alternative may actually be harmful in the long run. Certainly, many aquarists believe its use in tanks of Rift Valley cichlids can lead to bloating that doesn't happen when the correct mix of minerals is used instead. Similarly, if you're faced with high nitrate levels in your aquarium, you'd be better focusing on stocking density and frequency of water changes, rather than adding salt. As for treating Whitespot, there's a wide range of reliable medications out there that most people prefer to use instead.
If you stop adding the salt, you can solve this particular quandary, as after a few weeks' worth of water changes, the now unsalted aquarium water in the tank should be perfectly safe to use in the garden.
Related article: Frequently asked question on treating sick fish
Top tip
Brackish water fishes will appreciate the use of proper marine aquarium salt – rather than tonic salt – at dosages around 3-5g/litre and upwards, depending on the species.
Marine aquarium salt mixes are more than just sodium chloride. They also contain a whole variety of minerals such as magnesium sulphate and calcium carbonate that increase both the GH and the KH of the water, so buffering against the natural tendency of aquaria to become more acidic between water changes. This keeps the pH steady at between 7.5 and 8.2, depending on how much you use. This is exactly what brackish water fishes need to do well. Tonic salt, lacking these additional minerals, isn't nearly so good at buffering the pH in this way. But this brackish water is would still not be good for your plants no matter what salt you use.