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Salt And Cycles


Sudden changes in water parameters are not good for any aquatic creatures, and this includes the bacteria in your filter. Aquarium salt is a commonly used substance in quarantining and treating sick fish due to its effectiveness on many pathogens and relative mildness compared to some alternative medical treatments. Standard procedure is to increase the salt concentration by 0.01% every 12 hours to avoid shocking your fish, but the fish are not the only living creatures that need caring for in an aquarium. Denitrifying bacteria that live in your filter are also sensitive to changes in salinity, and are much more sensitive to increases in salt level. Acclimatising your bacteria to salty water requires smaller increment changes than the fish do, or a very stable and robust bacterial colony. When, for example, adjusting a tank to brackish or marine conditions the salinity is increased at much smaller levels than the fish could deal with to prevent losing the cycle for exactly this reason.

It should be obvious then that using salt while cycling your tank is so potentially dangerous. A colony that is still establishing itself can be wiped out or severely damaged by increasing the salinity level from 0 to 0.03% in the space of a day and a half. This is also why cycle bumps are relatively common in quarantine tanks and when treating with salt. In the majority of cases it is far more important that the water is kept free of nitrogen toxins and the bacteria is allowed to grow then that the fish is treated with salt, particularly with new tanks. If your fish definitely needs treating with salt, for example it is suffering from ich, it is crucial that you check your water parameters far more frequently than normally as it is more likely that ammonia and nitrite levels will rocket and water changes will be needed more often.

Sick fish are stressed fish, and stressed fish produce more ammonia. Combine this with a shakey cycle and you can see the problems.

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  • Regular Member

Do you have specific advice as to how salt should be added to least disrupt the cycle? Or is it a balancing act between the needs of the fish versus the needs of the bio bugs?


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  • Regular Member

If you have an established filter which is processing waste quickly and has been doing so for a number of months, you should have a large enough bacterial colony that they will cope even with die-off due to salting. The dead bacteria will feed the living ones as they rot but it can result in an ammonia spike due to less bacteria processing and an increasing amount of waste. This is why cycle bumps are so common in quarantine tanks even when using filter media from an established main tank. Personally I like to add the extra 0.1% salt every 24 hours instead of every 12, which will help to some extent but the main purpose of this topic is to emphasise that in a cycling tank using salt can do more harm than good, and I really don't recommend cycling and salting at the same time unless absolutely necessary as your cycle will not establish anywhere near as quickly with the salt in the water. In most cases it is more important to focus on the cycling as the ammonia and nitrites can be more toxic and harmful than the pathogen being treated with the salt, particularly if the salt is only being used as a preventative in quarantine. If the salt is not needed as urgently, it can be added in 0.05% quantities at a time which will help the filter bacteria, but will take longer to reach levels able to treat bacteria and fungus. (I have read somewhere of an actual number but I can't remember it off the top of my head, I'll try to find it!) If the salt is needed more urgently, for example for an ich breakout, just really keep an eye on the water parameters as there will be a cycle bump and you may have to do twice daily water changes to keep everything under control.

Hope this answers your question :) Let me know if you want any further clarification.

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