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mysiak 11-03-2018 09:39 AM

Chloride ions in a freshwater tank from anion exchange resin
Not sure where to put this topic, so please move it if it's in the wrong section.

It is often suggested to use salt (NaCl) as a cure or a preventive measure, with caution in planted tanks. It is probably well known fact, that some plants and livestock doesn't like too much of salt in the water. However I missed the explanation why exactly. Is it due to sodium, chloride or both ions?

The reason why I'm asking - at least in Europe it's quite popular to use anion exchange resins to selectively uptake nitrate from the tank water, mainly with tanks containing species sensitive to even small amounts of nitrates (discus for example) or breeding tanks. It allows prolonged schedule between water changes and keeps water pristine. Mechanism of working is that resin holds Cl- anions and replace them with NO3-, eventually exhausting itself and then it needs regeneration in strong NaCl solution (resin releases NO3- and replaces it with Cl- again). After recharge, resin is ready to do its magic again. So far, so good.

1 liter of "food grade" anion exchange resin can hold about 3 grams of NO3 (I don't have this number confirmed, but is probably accurate). Let's say that 1 liter of resin treats 200l of water for 2 months. So every 2 months we remove 3g of NO3 and add about the same amount of chlorides to the water. If I got the simplified math right, we increase chlorides by ~7.5ppm every month (and remove a bit of them by partial water changes). Effect should be similar to monthly dosing of salt in quite small amounts.

Now the million dollars questions - what happens with released Cl ions in the tank? Some of them are needed for life, but those are probably diminutive amounts contained in food or tap water already. The rest is being probably accumulated in water. What happens in the course of months or years? What is the side effect of excess amount of chloride ions? At which levels become chlorides dangerous?

The only study I could find is related to freshwater rivers/lakes which mentions as low as 3.1ppm and as high as 230ppm, depending on species etc.

Anyone here who could shed some light on this issue? :)

Gerald 11-06-2018 02:43 PM

Re: Chloride ions in a freshwater tank from anion exchange resin
Even the most sensitive aquatic plants are usually not bothered by 1 teasp (6 grams) of salt per 10 gallons, which is about 150 mg/Liter (60 mg/L Na + 90 mg/L Cl-). The amount you'd be adding via ion exchange is way less than that. And a planted tank probably doesn't have much if any excess nitrate to be removed anyway.

BTW, nitrate is a heavier ion than chloride (NO3 = 62 amu; Cl = 35 amu) so each 1.00 gram of NO3 removed would correspond to 0.56 g of added Cl-.

The 3.1 mg/L "tolerance indicator value" for brook trout in the Meador (2007) report was based on chloride field measurements at sites where brook trout were collected, NOT on the physiological tolerance of brook trout. Clean, clear headwater creeks, where brook trout usually live, have low chloride, but they CAN live in much higher salinity. There are even sea-run populations of brookies! Here's a link to the Meador paper, which the URI paper references:

The URI paper also mentions 160 mg/L chloride affecting salamander larvae in vernal pools. That's probably a more meaningful estimate of harmful chloride levels to sensitive freshwater species.

mysiak 11-07-2018 01:21 AM

Re: Chloride ions in a freshwater tank from anion exchange resin
Thank you very much Gerald, I was sure that I didn't get the weights correctly :)

Resin doesn't remove only NO3, I noticed that it also clears the water from tannins (and who knows what else). You are right that in a planted tank it doesn't make much sense to use it (at least 24/7). I will keep the resin at hand and use it only in case of emergency or when I want to clear up the water visually. But I am happy to know that even constant usage should not present health hazard to inhabitants or plants.

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