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TDS in RCS tank...HELP

1636 Views 6 Replies 4 Participants Last post by  en7jos
This is my first post on here, but I've learned a ton lurking around already, so let me say thanks for all the help!

To start, I have a 20gal tall tank that's been up and running for about 5 months now, ADA amazonia sub, Finnex Planted + 24/7 LED light that I run for 9 hrs/day (only a few hours of full light), canister filter with sponge on the intake, moderately planted, some dragon stone and one piece of driftwood. No CO2, dose ThriveS 3x/week (4 pumps at a time). I have, about 7-12 RCS (they're hard to count all at once) that have been in there for almost 3 months and I feed them 3x/week a very small amount of either powder or shrimp specific dry food. Only other fauna is some snails that hitched a ride on my plants.

Parameters: No ammonia or nitrites, nitrates at about 10-20 (very hard to distinguish with the API master test kit), pH has always been holding at the 6.0 level, GH of 4-5 degrees and KH of 0-1. I'm guessing my substrate is constantly buffering and I don't try to fight it. I just got a TDS meter last week and my initial reading was 279. From what I've read across multiple sites is that's on the higher end although it seems like there's constantly exceptions out there of people having success at the extreme ends.

I just installed an under the sink RODI filter and have done 2, 20% water changes so far this week. I remineralize with salty shrimp GH+ to get some stuff back in the water so my new water has a TDS of about 80, with the goal being to bring down my tank's TDS.

My question is this-what should I shoot for in terms of TDS/GH/KH/pH. What should my target TDS be given my GH, KH, and pH. Should I try to lower the pH of my new RODI water to match my tank water? My shrimp are active, eat, and swim around, but no breeding or babies yet.

Am I overthinking this? I feel like I am. I just want those little guys to be happy and have a nice tank for them.

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Hi @whm116,

Few additional thoughts / pointers for you in no particular order (I have similar shrimp tanks with Amazonia sub and have been doing lots of testing and measuring lately with SS GH+ minerals):

Firstly, calibrate your TDS meter. It might be new, but chances are that it is still "off". Get some TDS calibration "standard solution", cheap enough off Amazon etc, and calibrate. 342ppm NaCL solution is pretty standard. Before then, don't try and compare your TDS with anything (and going forward, take anyone else's TDS measurements with a large grain of remineralising salts unless you believe their meter is calibrated too!).

Having said that, have a read of my previous post in this thread which lists the TDS (ppm) readings that you should expect with Salty Shrimp GH+ at 5, 6 or 7 GH. Mt meter is a decent HM Digital model and calibrated last week, so I am as confident as I will ever be about ppm's, but still please don't take them as exact. I go with 7GH like what @Zoidburg said above for my RCS:

Remineralising to 7 dGH using 1.75g of SS GH+ per 10 litres of RO water gives a TDS of about 150 ppm.

The old water in your tank will be at a much higher TDS than your water change water because of all the other stuff you add to your tank (e.g. fert's, food, etc). My RCS tank is currently sat at about 300ppm (despite w/c's with 150ppm water as above) but the shrimp seem happy with that. When @Zoidburg said "TDS should be between 125-150" I think he means your fresh, newly remineralised RO water, not the tank itself.

Stability is arguably more important than any specific water parameter, so don't try to change anything suddenly to match what someone else says your tank should be at. RCS are pretty adaptable so don't fix it if it aint broken. 20% water changes *could* be on the high side depending upon how much it causes the tank water parameters to fluctuate. I've been aiming for 10% changes, maybe just 5% if nothing is too high.

The TDS of your tank water is GH + KH + other stuff. If your tank TDS is high, you need to know whether the tank water is too hard (GH and/or KH high) or just "dirty" (i.e. correct GH / KH but too much other stuff). My point being that you could do water changes with pure RO to bring the TDS down, but a TDS reading doesn't tell you what the "staff" in the water is, and you could theoretically end up with a tank with a wonderful TDS of 150-200, but with 0 GH and 0KH (i.e. just "other stuff").

One degree of hardness = 18ppm roughly. So if your water is 7 GH and 2 KH then that's (7 + 2) x 18 = 162ppm. If you're TDS pen reads 200 ppm then you know that there is 38ppm of "other stuff". TDS readings by themselves are not helpful unless you know how much of ppm is made up of necessary minerals (GH and / or KH) and hence how much is the "other stuff" (fert's, food, dechlorinator, etc, etc).

Assuming you're using the API pH test kit, then pH 6.0 is the lowest reading on the scale, but the actual pH could be 4 or 5, you have no way of knowing. Not a problem, but just helpful to realise that the test result is quite possibly less than 6! Also, if your water is 0 KH then it seems that most pH measurements could well be off anyway.

If you want to get more accuracy from API GH / KH test kits, use twice the amount of water then each drop equates to 0.5 degrees hardness (rather that 1). Sometimes useful when you are initially working out how much you need to add to your w/c bucket.

I know the pot says that the SS minerals dissolve "almost immediately", but I have found that you actually need to give them time to mix and dissolve properly before you take GH or TDS readings. If you do this immediately, the water may read about 1 dGH less than it actually becomes an hour or so later. Best to mix up water day before water change if possible and leave with a powerhead / pump / airstone running to ensure minerals properly and fully dissolved. But with the SS, I've found that about 1/2 hour is usually enough for TDS to settle down.

Hope there's some useful pointers here. Happy shrimping!
James =)

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1) Calibrate my new TDS pen
2) Do smaller water changes (10%) with RODI water and remineralize with salty shrimp GH+ to get 7 degrees hardness and a target TDS of the new water of 150 (after letting the stuff dissolve for a while).
Yes, but no. Completely disregard any TDS figure anyone has ever told you. Remineralise with SS GH+ until you get a GH of 7 as measured with API liquid test kit (using the double water volume trick for 0.5 GH accuracy if you want, but probably not necessary). Best way is to get a set of small weighing scales (just make sure they're accurate to 0.1g if possible) and do this by weight, but you could also get some (very) small measuring spoons (or any small spoon, e.g. Starbucks coffee stirrer) and work out how many spoonfuls you need to add to get 7 dGH.

Once you have a reliable way to mix up your 7 GH water by adding X grams or Y spoonfulls of SS GH+ per Z litres of water, only then measure the TDS with your pen. Then you can either:
a) be proud at how well you calibrated your TDS pen because your answer is just like what you read somewhere
b) realise just how pointless TDS measurements actually are as your freshly calibrated reading is still so different to what you've read

You can then use YOUR TDS reading to gauge future remineralising, but since you've already accuartely worked out how much minerals to add by weigh or spoonfulls to get your desired GH, this is now pretty pointless. The real use of a TDS pen is however to then monitor the TDS of your tank water and see how much greater the reading is than that of your freshly remineralised RO water. In time you'll get a feel for when the TDS is rising and a water change is necessary. For example, you might decide that you do water changes whenever the TDS reaches X ppm.

Regarding Equilibrium, it does give a high TDS reading for a given GH compared to shrimp-specific minerals because it has lots of other stuff in it that is great for plants. But for shrimp, we typically want "pure GH" minerals without any of the extras to keep TDS low.

I wouldn't worry about what exactly the pH is if you're still getting the pH 6.0 minimum reading on your API liquid tests. Amazonia apparently starts of buffering down to about pH 5.5, but like you, I've only ever been able to measure that it's 6.0 or below with the API test. I bought a decent pH pen, but that doesn't help as it responds incredibly slowly and is not at all accurate at low KH. You could get one of the other brands of liquid test kit that @Zoidburg mentioned, but if resolution is 0.5 pH then this will only tell you that it is either 5.5 or 6.0. You pretty much know that already as Amazonia doesn't tend to buffer below 5.5 (apparently) and you've measured that it is not more than 6.0.

Just test occasionally to see when the pH starts to creep up, which it will do sooner or later as the buffering capacity of the substrate is exhausted. At that point you can start thinking about adding driftwood / leaves / blackwater extract type stuff and all those "natural" ways to keep the pH low, if you want to, or maybe let it rise a bit for RCS. Don't add buffering products to fight the substrate.

This is a useful explanation of pH and hopefully explains why the pH of your RO water doesn't matter too much if it's at 0KH:
and also

Happy reading! James :nerd:
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