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Moving to an RO / Tap Water Mix

Aeropars

Member
Joined
9 Jul 2007
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802
Location
Leicester
Hi all,

Following years of jealousy over those with super soft water, I've decided to get me an RO unit. Not specifically for aquarium use butwill be heavily weighted towards it. Having done some reading yesterday I find myself a little confused. Back when i used to test my water, it was all about GH/KH but this seems to have moved to a TDS model now. Naturally, i'll be getting a TDS meter as part of the setup given they are inexpesnsive.

I live in the East Midlands which is very hard water from what i'm told but what i'm not sure of is just how hard. While a curse to some extent, it seems a blessing with RO because i can just split the water changes with tap water to lower the overall hardness.

So the question I have is, what is a good softness/TDS level for general plant and fish health? Do i need to be concerned with GH/KH measurements?

If there are any good beginner guides to TDS that i may have missed, more than happy to be pointed in the right direction!

Thanks
 

bazz

Member
Joined
24 Jan 2009
Messages
339
Location
Lincoln
Morning,
I'm in Lincoln, not too far away and my tap (from memory) is roughly 630 μS, 300 TDS, 16GH and 12 KH. I use remineralised pure RO to give me 5 GH and 1 KH which equates to about 100 TDS (which increases over the week using EI) or 230 μS.
I would mix RO with 20 to 25% tap, which is what I have done in the past and not worry too much about the GH and KH levels thereafter, I successfully kept tetras and corys (spawned) in these conditions but I can't remember the parameters.
Hope these figures will give you an approximate ballpark to play in.
Cheers!
 

dw1305

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UKAPS Team
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7 Apr 2008
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14,903
Location
nr Bath
Hi all,
Back when i used to test my water, it was all about GH/KH but this seems to have moved to a TDS model now. Naturally, i'll be getting a TDS meter as part of the setup given they are inexpesnsive.
There isn't any <"empirical connection"> between water hardness dGH (the amount of multivalent cations), alkalinity (the amount of proton acceptors, usually called dKH) and electrical conductivity (~the amount of ions in solution, often <"called ppm TDS">), but in most situations they are linked, because all three relate to the amount of dissolved limestone (calcium carbonate (CaCO3) (as Ca++ and 2HCO3-)) content of the water.
If there are any good beginner guides to TDS that i may have missed, more than happy to be pointed in the right direction!
@_Maq_ has written an article that looks and hardness, alkalinity and conductivity <"Some handy facts about water">.

cheers Darrel
 
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Aeropars

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Thread starter
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9 Jul 2007
Messages
802
Location
Leicester
Thanks both.

@bazz Interestingally, i thought i was in a very hard water area but Severn Trend suggest moderatly hard. Just over 10 german degrees which is lower than i expected. None the less, i feel i'll benefit from RO. Not least because i'll use it to brew beer with as well :D
 

GHNelson

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14 Dec 2008
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5,705
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Hemel Hempstead
I usually add 25% Tap water to 75% RO!
I leave the RO water overnight to get it up to room temperature, then add the tap water in the morning/afternoon.
 

MichaelJ

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9 Feb 2021
Messages
2,204
Location
Minnesota, USA
Hi @Aeropars,
it was all about GH/KH but this seems to have moved to a TDS model now.
Not really. GH is a very specific measurement of the concentration of Calcium and Magnesium (some traces such as Iron adds to it but its insignificant) whereas TDS measures all minerals wholesale (including Ca/Mg) that carries a charge (cations and anions).
So you can conceivably have extremely soft water in GH terms and still have a very high TDS level - say if you water contains a lot of Nitrate, Phosphate, Potassium, Sulphate, Chlorides etc. However, you cant have low TDS (below 100ppm, 200 uS) if your water is extremely hard - for instance, a +15 GH all from Calcium.

Now, TDS or Electrical Conductivity is something some of us likes to talk about a lot :) In short: Keeping TDS low is beneficial for soft water fish as it helps with osmotic regulation - by far most of our soft water fishes originates from habitats that contains very low amounts of minerals (very low EC/TDS). Moreover, measuring TDS is very helpful to monitor the stability of your tank. While TDS is not directly measuring organic waste, a lot of byproducts from waste breakdown will show up as TDS. If you over time see TDS creep there is a good chance this is due to waste buildup or perhaps "overdosing" or accumulation of certain fertilizers, leaching hardscape etc.

And you can use the TDS meter as a sanity check for your water prep. Say when you remineralize your RO water and adding all the compounds (Ca Mg etc.) you get a pretty good ballpark idea whether you've done it right. For instance, if you remineralize to a target of say 5 GH from CaSO4/MgSO4 in a 3;1 ratio and measure ~70 ppm (140 uS) that would be fairly accurate (the Sulphate with CaSO4/MgSO4 adds a lot to the TDS) and you can use that as a guiding pole that your getting it right over time... or as in your case when mixing tap/RO water. I've messed up a couple of times with my remineralizing and wouldn't have caught my mistake without a TDS meter.

So the question I have is, what is a good softness/TDS level for general plant and fish health? Do i need to be concerned with GH/KH measurements?
In general, keeping GH low (2-6 GH is a good range) promotes nutrient uptake in plants and benefits soft water fish species - invertebrates such as shrimps generally require a bit more - say 5-6 GH to build their exoskeleton. Very few plants strictly requires lower GH than this and very few plants require higher GH. As far as a "good" TDS level goes... With RO water as a starting point, you should be able to maintain 75-125 ppm (150-250 uS) with a low GH/KH and still have enough headroom to provide plenty of macro nutrients (NPK) if your following more mainstream fertilizer approaches.

KH: There seems to be very little reason for aiming at a specific KH level as long as its low. Many of us around here run our slightly acidic soft water tanks at ~1 KH. Due to the weak buffering capacity at low KH (lower KH equals lower resistance to change in pH), it is easy to acidify the water by natural means such as by adding botanicals (almond leaves etc.).

Naturally, i'll be getting a TDS meter as part of the setup given they are inexpesnsive.
Yes, get one that can measure EC as well as TDS. They all measure EC internally, but some products only displays the TDS. If the one your buying will only display TDS make sure you know the EC to TDS conversion factor.

Cheers,
Michael
 
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