RO Mix: SO4 and Cl? - The Planted Tank Forum
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post #1 of 8 (permalink) Old 05-29-2020, 02:40 PM Thread Starter
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Question RO Mix: SO4 and Cl?

Finally making the transition to 100% RO water, and now have sooo many options for water parameters I am a bit unsure of where I want to land.

Tank is a little high-tech 15 gallon set up with ADA Aquasoil. I have some picky plants that (I think) like low dKH and high Mg - Rotala macrandra mini and Rotala macrandra variegated as well as Ludwigia white.

I am remineralizing my water to 25 ppm Ca: 12.5 ppm Mg: 25 ppm K, with dKH of 1.8, and dGH of 6.4 using CaCl2, MgSO4 and KHCO3. I feel like I should have a higher dGH and Ca above 30, however, with my remineralization mix I already have Cl at 45 ppm and I am worried about Cl- toxicity. (S is very low at 16 ppm with that mix).

I know that Chloride from CaCl2 does not have the toxicity of Chlorine from our chlorinated water supply, however I have read some scientific articles that show toxic effects of build up of Cl- as low as 20ppm, and as low as 8 ppm for spawning fish (granted, the article was talking about spawning trout which I definitely will not have in my tank).

I think if I am going to increase Ca and Mg, I will buy some CaSO4 as toxicity levels for SO4 seem to be super high (unless you have anaerobic areas in your tank and Sulphate gets converted to toxic Sulphide which is a niggling worry looking at the four inch deep substrate at the back of my tank). Although not sure if I need to increase Ca and Mg???

Looking for 1) general advice, 2) your thoughts on Cl and SO4 toxicity and 3) What mix do you use?
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post #2 of 8 (permalink) Old 05-29-2020, 04:11 PM
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Curious how others will chime in here. And also, what made you start using CaCl2?

Not that it will help you that much since I don't have R macranda but I'll throw my specs into the mix just in case.

10gal tank with Amazonia type soil using RO exclusively. I use CaSo4 and MgSo4 at a 27:10 ppm ratio to achieve ~6dGh. I also haven't gotten away from NaHCo3 yet that I dose to target 1dKh.

SO4 toxicity requires a lot. Not sure you can get to toxic levels of SO4 with normal maintenance routines and dosing regimens. Of course I'm sure there is someone out there that has done it.

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post #3 of 8 (permalink) Old 05-29-2020, 04:25 PM
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Chloride (Cl-) is inert and non-toxic. However, chloride levels are used as a measure of salinity. Sea water, for example, has 19,353 ppm chloride, and freshwater typically contains less than 100 ppm.

Those species that require < 20 ppm Cl- simply require water with very low TDS. Salinity tolerance is expressed in chloride levels because, at one time, chloride was the easiest to measure. There are better methods now.

Sulfate (SO4) is also inert and harmless at low levels. Sea water contains 2,712 ppm sulfate and freshwater typically contains less than 80 ppm.
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post #4 of 8 (permalink) Old 05-29-2020, 05:28 PM
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I know lots of folks using RO water, and pretty much everyone uses CaSO4 & MgSO4 to boost GH.

I keep mine at Ca:Mg 30:15, but really your 25:12.5 or really anything in that range should be fine.

As to KH, I dose mine to 1.0 dKH. But again, anything in that very low range should be fine.

Do keep in mind the K being added from KHCO3. I use MgNO3 instead of KNO3 to keep K levels in line.
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post #5 of 8 (permalink) Old 05-29-2020, 07:07 PM Thread Starter
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Quote:
Originally Posted by pauld738 View Post
And also, what made you start using CaCl2?

Solubility. I think CaSO4 can be a pain to dissolve?
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post #6 of 8 (permalink) Old 05-29-2020, 08:35 PM
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Oh, got it.

I always use a power head/sump pump recirculating water when adding Ca:Mg to RO, be that a 32gal trash can or a 5gal bucket. Then run off and do something else. Never had to wait for it. But then again I'm not sitting there watching it dissolve

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post #7 of 8 (permalink) Old 05-29-2020, 10:08 PM
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I target K and Ca at 20ppm and Mg at 10ppm.

With a weekly 50% w/c, I initially replenish K and Ca based upon a KH target designed to hit a ph of 6.5, given my current CO2 dosing. I do this based upon the charts, you’ve probably seen them, showing optimal uptake of ferts in the pH 6.5 area AND because 6.5 is about as low as I can go before my ramshorns start to show the effects of acidic water on their shells. So, I use KHCO3 to do this. This usually means that the K target of 20ppm is reached before the necessary KH target.

To complete the KH targeting, I add the necessary bicarbonate with CaCO3, which also adds a good amount of Ca. With a pH of 6.5, the CaCO3 dissolves overnight. If I need more Ca to reach my Ca target of 20ppm, I add it with calcium gluconate (I get enough SO4 from my MgSO4).

I only add about .4ppm Cl to my tank, daily, through an autodoser, which is an inconsequential amount of Ca from the CaCl. I’ve read enough to convince me that staying below 7ppm is possibly the safest thing to do and I do worry a little about the accumulation effects, despite 50% w/c’s. My plants are certainly not complaining about the Cl dosing level. In fact, I recall reading, somewhere, that Cl should be dosed at the same rate that Fe is dosed - true or not.

Summing up; I add KHCO3, MgSO4 and, if needed, Ca gluc to the barrel holding my RO/DI replacement water, the CaCO3 to the tank right after the w/c and the Cl daily through the autodoser along with urea.
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Last edited by Deanna; 05-29-2020 at 10:34 PM. Reason: -
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post #8 of 8 (permalink) Old 05-30-2020, 09:08 AM Thread Starter
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Thanks everyone for the comments. This is super helpful!
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