Questions About Na, Ca, K, Mg, Carbonates & Nitrates

I can't answer all of your questions, but it appears that you haven't done a fully planted freshwater tank, so I'll inform you that the best way to remove nitrates is not with "denitrators" or "huge algale scrubbers", but rather with true aquatic plants. If you fully stock a tank with plants (using some fast growers as well) you generally will fully use up all of the nitrate in the tank before the week is up. This becomes more true if the lighting is really high, as long as the tank isn't overstocked. As far as for "good salinity level" for FW tanks, it is best to just use tap water with dechlorinator. Don't add any salt! You only need to add salt for brackish water tanks, which aren't what you are asking about. Most plants don't do well with salt anyways, although a few are more tolerant. That's all I can post definitively on, I'll leave the rest to the chemists!

I'll take a stab at a few of these...

1) Nitrates are produced by the animals in our tanks, accumulate, and unless we use those goofy denitrator coils, or humungous algae scrubbers, we have to change our water to remove them.

True plants will also consume nitrates. A variety of plants within the tank can be used to consume them, or fast growers can be harvested to actually export the nitriates from the system.

2) KH, or carbonate hardness is, a) a measure of CO3 ions of various elements, and b) consumed by plants and animals to make cellulose, shells, and skeletons.

It's also utilized by normal biological activities--bacteria process KH. Read this, especially the second post: [/url]http://www.aquariacentral.com/forums/showthread.php?t=35281[/url]

3) To maintain KH, it must be dosed regularly. A commonly used compound is Sodium Bicarbonate, or baking soda.

Partially true. It must have a source, but there are several options. For low KH water, it may be supplemented with adding crushed coral, baking soda, or something similar that will add the calcium. If the source water isn't low KH, frequent regular water changes are typically sufficient to maintain steady KH levels in most setups.

Ok here are the questions. They sort of play off each other more or less in order, so would be best to read them all before replying one at a time.

1) Generally speaking, which nitrate compounds are we trying to get rid of? I am assuming they include those of Na, Ca, K, and Mg.

Nitrate isn't reduced in planted tanks through artificial means--it's supplemented to feed the plants. Various forms are used to supplement trace elements that are beneficial to plants--iron, magnesium, etc. In an unplanted tank, nitrates themselves are not harmful in low to moderate levels, but are a good indicator of other pollutants that aren't easily tested for, so they can be used as a canary.

2) Why dose KNO3 or CaNO3 if nitrates are a problem...? This seems counterproductive to me.

I think you're mixing treatments...But if nitrates are low enough to impede plant growth, dosing them in a manner that also provides other beneficial elements will help the plants.

3) Why not dose KCO3 and/or CaCO3 instead? Since we want K and Ca, AND carbonates, this seems to be ideal.

It depends on what's short--if plant growth is carbon limited, either is good, but if the plant is starving for nitrates and just fine for carbon, dosing more carbon won't help.

4) As KH is consumed, does the whole molecule get used, or just the CO3, leaving the other elements behind? If not, what happens to the rest? In the case of sodium bicarbonate, what happens to the Na?

Sodium is also used by most animals and plants, in small amounts, I'd suspect that it just gets used up, like the carbon, iron, etc. Reference Happy's article again for how it breaks out--the NA doesn't form NaCl.

6) NaCl would raise salinity. Does the term "salinity" also include other salts such as KCl and MgCl...?

No, salinity isn't used to reference other salts in aquaria, not that I've ever heard, anyway.

7) What is a good salinity level for a freshwater tank?
Planted tanks should have a specific gravity very near 0 ppm.

8) I have read that GH is a measure of magnesium and calcium in the water, and is not consumed like KH - it "just is". But what compounds of Mg and Ca is it?? Chlorides? Carbonates? I haven't yet seen an explanation of that.

Again--reference happy's article. It covers this in much better detail than I can.

spinjector said:

2) Why dose KNO3 or CaNO3 if nitrates are a problem...? This seems counterproductive to me.

3) Why not dose KCO3 and/or CaCO3 instead? Since we want K and Ca, AND carbonates, this seems to be ideal.

Thanks!

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Just to clarify, either KNO3 or Ca(NO3)2 (I'm thinking of majoring in chem, so I'm a stickler for nomenclature ) can be used to dose the NO3. When using KNO3, you're adding K+ ions as well as the NO3- ion into the water, both of which are used by plants.

If your water is soft, then you may want to dose Ca(NO3)2 instead, as this will also release some calcium into the water (needed for "non-wrinkly" plant leaves). However, you should be aware that because of the nature of the Ca+2 ion, there will be 2 times as much NO3 that is released into the water (compared to KNO3).

As for the K2CO3 (again with the nomenclature) and the CaCO3, the K2CO3 is useful if you want to put in more K+ into the water. However, be aware that the CO3 will combine with any hydrogen in the water to form carbonic acid (H2CO3) which then breaks down into water and CO2.

As for the CaCO3 it is insoluble in water, and I don't think many of us want a white gunk sitting at the bottom of our aquariums. (For those that were curious, Ksp of CaCO3 = 3.8 *10^-9)

Minor addition to otherwise excellent responses.

CaCo3 Calcium carbonate is precisely what we are dissolving in water when we add crushed coral to our filter. It does not easily dissolve in water, but does do so and fairly quickly in lower Ph levels. I dose Dolemite which is calcium Carbonate and magnesium carbonate but is generally just listed as CaCo3 for aquarium folks, I also use crushed coral and limestone at times. both of which are forms of calcium carbonate. with co2 injection, my ph stays below 7 and the coral, dolemite, and limestone dissolve fairly rapidly by all rights.
Dave

spinjector said:

Oh yes I have... My tank is quite lushly planted in fact. I was speaking about the nitrates in the hypothetical sense. Since I have a large fish load, my nitrate exceeds that used by plants, so I do have to keep an eye on it. I have a Coralife 28 watt 6500K CF on it; a 20 gallon tank. I do not add salt, I was asking because what we call "freshwater" is not "absence of salt", but rather on a scale somewhere between distilled water and sea water. I was wondering what a good spot was - just for reference, not because I had plans on fiddling with it. Thanks!

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Well, even this is a difficult one to answer with the "shotgun" method of giving one broad answer to be "correct". The truth is that the salinity (along with KH, GH, and other TDS in the water column is variable, and different plants grow in quite a broad range throughout the world. Some fish live in such soft acidic waters which have almost to TDS in them. Others live in what we call "liquid rock". Of coarse many people add salt to their tanks containing fish like Mollies, because these fish can live in waters similar to brackish environments (very high TDS). I would say the best measurement to worry about would be TDS, not salinity. Because there are fish who can live well in water straight from an RO unit, but you just have to keep up with PH crash possibilities, then. Consider the broad range from nearly pure water from an RO device (around 20 TDS) to a tank in which you have a 9.0 pH, and actually add chemicals like "Cichlid salts" (ie. Frontosas and Tropheus), or even brackish-tolerant freshwater fish (some Acaras, Mollies, and Green Severum, etc.) and you will have a wide range of TDS fish live and thrive in, while still being considered Freshwater fish. Many different plants tolerate different water hardnesses, as well. Mirofillum and several fine-leaved plants do well in low TDS, while Java Fern and Mangroves do really well in up to brackish water.

If you are wondering if your fish will thrive in your water, give us some species of fish (or plants) and what your water pH, KH, GH, and other levels are. We can answer much more productively with this info.

I do not add salt, I was asking because what we call "freshwater" is not "absence of salt", but rather on a scale somewhere between distilled water and sea water. I was wondering what a good spot was - just for reference, not because I had plans on fiddling with it. Thanks!

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As a general rule the Na and CL levels in freshwater are very low and usually fairly similar to the levels we find in our tap. These are also borderline on being non-essential as a rule with most freshwater fish and plants, so it's something no one pays much attention to. Beyond what comes from traces mixes, or common additives. Cl is added through many different avenues, sodium is seldom added purposefully, and how they bond, and or seperate in water is beyond my basic knowledge of chemicals. A lot of natural soft water contains none, but even the rift lakes contain very little Na and CL in the sense of teaspoons per gallon.
Sorry I don't have a more precise answer for you on this.

Since I have a large fish load, my nitrate exceeds that used by plants, so I do have to keep an eye on it.

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Any time that nitrate production exceeds plant uptake, water changes are really the only good control. when we attempt to increase plant growth we are locked into routine water changes to keep ferts and TDS levels in balance. So basically increasing plant uptake really isn't the good long term answer. the tank experiments that have been done to use plants instead of water changes longterm fall under specific guidlines. primarily light dosing, small bio0loads, and trimming of plants to remove excess nutrients. This from what I've read can be successful if done properly, but doesn't fall under the conditions of most of our tanks.
Dave