Gh, kh, and tds explanation?

Griffboy123

I’m very confused about gh, kh, and tds. I used RODI water to fill up my tank while it was cycling. Now I’m reading that just RODI water has no minerals and will kill fish and plants if I only use that… I have plants in there for around two weeks and all are fine and some are growing a lot. The only thing I have been adding to this 10 gallon tank is dr tims ammonia and flake fish food. My cycle is complete now but I’m just confused as to what I should do about this gh, kh, and tds levels which should be at zero because I’m using RODI water. Can anyone explain how to fix this?
 

RayClem

GH is general hardness. It is the measurement of the amount of bivalent metalic ions in the water. That includes such things as calcium, magnesium, iron, and others. It can be expressed in parts per million (ppm) or in degrees of hardness. One German degree of hardness is equal to 17.8 ppm.

To further confuse things, in the USA, water companies normally report water hardness in grains per gallon. One grain per gallon equals 17.1 ppm, just slightly less than the German degree.

Some aquarium water tests report in German degrees. For example, the API General Hardness test is design such that 1 drop of test solution per 5 ml sample represents 1 German degree of general hardness. Some test strips report general hardness in ppm. If you need to convert between them, use the 17.8 ppm/degree factor.

KH or karbonate hardness (from the German spelling of carbonate), is a measure of the alkalinity of the water. That is the reserve available to prevent swings in pH. If the KH (alkalinity) is low, there is little reserve to keep the pH stable. The alkalinity test is designed such that the endpoint is a pH of 6.0. Anything below a pH of 6.0 will have an alkalinity of zero. Carbonates stabilize pH by conversion into Carbon dioxide when the pH drops. You may have observed this phenomena by dropping acid such as vinegar onto limestone. The acid reacts with the limestone (calcium carbonate) and releases carbon dioxide, which creates a fizzle sound.

TDS is total dissolved solids. The proper measurement of TDS would be to measure the volume of a solution and then measure the weight of a residue left behind when the water is evaporated. However, this method is problematic as calcium carbonate when heated will release CO2. It is also quite cumbersome. Fortunately, ions dissolved in water produce a solution that is electrically conductive. The electrical conductivity of the solution as measured between two probes of known separation will be approximately proportional to the total dissolved solids. That is how TDS meters work. They are actually conductivity meters that use a calibration to report TDS.

I use a TDS meter to measure the water output from my reverse osmosis system. I know that when the TDS exceeds 20 ppm, it is time to replace the membrane in the RO unit. In an aquarium, TDS is much less useful as you have no idea whether the dissolved solids are monovalent ions such as sodium and potassium, bivalent ions such as calcium and magnesium, or various anions such as chloride, sulfate, carbonate, etc.

The important measurements to operation of an aquarium are General Hardness, Alkalinity, and pH. As long as those values are where you want them to be, the TDS value is useless.
 

Griffboy123

that was really detailed and explained really well and thank you for that. So I don’t really need to worry about tds it seems… now for the kh and gh I’m assuming I should test that? (What would the optimal gh and kh be if I do need to test it?) I know my ph has been stable so far at 7.2 as I’ve been checking it throughout the 2 weeks it’s been up.
 

RayClem

that was really detailed and explained really well and thank you for that. So I don’t really need to worry about tds it seems… now for the kh and gh I’m assuming I should test that? (What would the optimal gh and kh be if I do need to test it?) I know my ph has been stable so far at 7.2 as I’ve been checking it throughout the 2 weeks it’s been up.

The main things with GH, KH and pH is that it should be stable. The exact number is not terribly important. There are some fish like guppies, mollies, platies, and swordtails from Central America that like hard, alkaline water, but they will often acclimate to softer water acloser to neutral pH. African Rift Lake cichlids (mbuna) come from lakes carved from limestone. They also like hard, alkaline water.

South American rivers are frequently very soft and acidic due to the tropical rainforests. If you aim to breed these fish, soft acidic water might be helpful, but today many of the fish in the aquarium trade are captive bred in water that is moderately hard and slightly above neutral pH. Thus, as long as your water is not extremely hard or extremely soft and very acidic or very alkaline you should be OK.

While more experienced fishkeepers sometimes try to adjust the pH, hardness, and alkalinity of their water, but this is an advanced technique that is best avoided by newcomers. As long as your tap water is not extreme (like mine is), you should be OK using your tap water and a dechlorinator/water conditioner. Just do frequent water changes to keep the water properties stable.
 

Griffboy123

The main things with GH, KH and pH is that it should be stable. The exact number is not terribly important. There are some fish like guppies, mollies, platies, and swordtails from Central America that like hard, alkaline water, but they will often acclimate to softer water acloser to neutral pH. African Rift Lake cichlids (mbuna) come from lakes carved from limestone. They also like hard, alkaline water.

South American rivers are frequently very soft and acidic due to the tropical rainforests. If you aim to breed these fish, soft acidic water might be helpful, but today many of the fish in the aquarium trade are captive bred in water that is moderately hard and slightly above neutral pH. Thus, as long as your water is not extremely hard or extremely soft and very acidic or very alkaline you should be OK.

While more experienced fishkeepers sometimes try to adjust the pH, hardness, and alkalinity of their water, but this is an advanced technique that is best avoided by newcomers. As long as your tap water is not extreme (like mine is), you should be OK using your tap water and a dechlorinator/water conditioner. Just do frequent water changes to keep the water properties stable.
I see. Okay, I’ll see if my tap is extreme and then go from there. It does seem easier to use prime rather than buying rodi and dosing it. I will do 20% water changes weekly as well. Thank you a lot for the this you really helped a lot!
 

MacZ

GH is general hardness. It is the measurement of the amount of bivalent metalic ions in the water. That includes such things as calcium, magnesium, iron, and others. It can be expressed in parts per million (ppm) or in degrees of hardness. One German degree of hardness is equal to 17.8 ppm.

To further confuse things, in the USA, water companies normally report water hardness in grains per gallon. One grain per gallon equals 17.1 ppm, just slightly less than the German degree.

Some aquarium water tests report in German degrees. For example, the API General Hardness test is design such that 1 drop of test solution per 5 ml sample represents 1 German degree of general hardness. Some test strips report general hardness in ppm. If you need to convert between them, use the 17.8 ppm/degree factor.

KH or karbonate hardness (from the German spelling of carbonate), is a measure of the alkalinity of the water. That is the reserve available to prevent swings in pH. If the KH (alkalinity) is low, there is little reserve to keep the pH stable. The alkalinity test is designed such that the endpoint is a pH of 6.0. Anything below a pH of 6.0 will have an alkalinity of zero. Carbonates stabilize pH by conversion into Carbon dioxide when the pH drops. You may have observed this phenomena by dropping acid such as vinegar onto limestone. The acid reacts with the limestone (calcium carbonate) and releases carbon dioxide, which creates a fizzle sound.

TDS is total dissolved solids. The proper measurement of TDS would be to measure the volume of a solution and then measure the weight of a residue left behind when the water is evaporated. However, this method is problematic as calcium carbonate when heated will release CO2. It is also quite cumbersome. Fortunately, ions dissolved in water produce a solution that is electrically conductive. The electrical conductivity of the solution as measured between two probes of known separation will be approximately proportional to the total dissolved solids. That is how TDS meters work. They are actually conductivity meters that use a calibration to report TDS.

I use a TDS meter to measure the water output from my reverse osmosis system. I know that when the TDS exceeds 20 ppm, it is time to replace the membrane in the RO unit. In an aquarium, TDS is much less useful as you have no idea whether the dissolved solids are monovalent ions such as sodium and potassium, bivalent ions such as calcium and magnesium, or various anions such as chloride, sulfate, carbonate, etc.

The important measurements to operation of an aquarium are General Hardness, Alkalinity, and pH. As long as those values are where you want them to be, the TDS value is useless.
Just a few annotations:

GH consists just of AEMs, i.e. calcium and magnesium. It is basically only made up of these two, often in compounds, with barium the third biggest percentage. The few percent left are the other AEMs. Iron is not part of that group.

KH as we measure it (with drip tests I mean), is acid capacity as an indicator of carbonates. But KH has long been established, but it's worthwhile to know that we don't actually test for it.

TDS measuring in aquaria can be of use in softwater tanks with significantly less than 100 mg/l TDS as their maximum.
 

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