Calculate ratio of well water (hard) and rain water (soft)

Can someone please help me calculate the correct ratio of my well water to my "tap" water to maintain an appropriate KH of 4-8? (I have neon tetras, angle fish, and cory cats in 55 gallon tank) .
My "tap" water is collected rain water from our roof. It is filtered twice before UV sterilization (it is our drinking and household water). It has a pH of 5.5, a GH of less than one drop per the Hagen Nutrafin Test. This calculates to less than 20 ppm and less than 1.12 dGH. The KH tests at less than 1 drop which calcutales to less than 10 ppm and less than .56 dKH.
The well water is very hard. The pH tests at somewhat higher than 7.5. The GH test required 21 drops (Hagen Nutrafin test) which calculates to 420 ppm and 23.52 dGH. The KH test required 38 drops which calculates to 380 ppm and 21.28 dKH
Given we know the parameters of both sources of water, I would think there should be a way to mathematically calculate the proper ratio.

Thank you.

 

Because you have no appreciable amount of GH or KH in the rain water, a 19% ratio should provide you with about 4dkh or 72 ppm. 380ppm x 19% = 72ppm This would equate to about 3 cups hard water to 13 cups rain water to make 1 gallon of mixed water. Try mixing a gallon using those quantities and test the KH to verify the result. A 38% ratio (6c. hard to 10c. rain) should provide you with about an 8dkh (144ppm) water mix.

Let me know how you make out.

 

Thank you so very much. I will try the ratio tonight. Do you know if the mixing of hard to soft water is linear? Will twice the amount of hard water make the mix twice as high in regards to dKH?

 

That is a very good question. As is obvious in my calculations, my gut answered, "it should be", but the proof is in the pudding and because I've never run that particular type of test comparison before, I decided to get off my butt and go do it. I've learned that I have more than a little exhausted my RO membrane and have to get busy and change it, as I've discovered it now has about 30ppm KH in it, but for our test it will still serve it's purpose.

My well water KH is currently at 390ppm. This number does vary with the seasons, but that is it's current reading.

The 3 cups hard water to 13 cups RO measured in at 100ppm (5.5 dkh). I'm usually around the 70ppm (4dkh), but the RO's current KH level will be raising this a little higher than usual.

In a second bucket, I tested 6 cups hard water to 10 cups RO and it measured in at 170ppm (9.5 dkh). Again, the KH level in my RO water will affect this result.

Maybe with absolute pure water (0 KH), the results may be linear but it is not linear when it's mixed with a soft water that still has some KH value in it. It is something I'll have to test again after I get the membrane changed to see if the results are linear under those conditions. For our purpose, I think it gives you a fairly good ball park. Your rain water may be anywhere from 0ppm, to 10ppm KH, and your hard water is slightly less hard than mine, so I still anticipate your results using these amounts should land you in the playing field you are looking for.

If you're willing, I'd like to see your test results if you perform these same tests.

 

Hmmmmm, I just carefully blended 5 cups of well water to 13 cups rain water and the dKH calculates as 2.8 (The color changed with the 5th drop, multiply that by 10 to get 50ppm and multiply that by .o56 to get the dKH of 2.8, So I went back and retested the well water which today tests at 24 drops which is a dKH of 13.44. I brought the bucket of water inside 3 days ago and put the recommended amount of "Tap Water Conditioner" which is supposed to remove the chlorine (we do chlorinate the well water), detoxify the heavy metals, and break the chloramine bonds. I have also been running air thru it via an airstone. Did that drop the KH? This is the same bucket of water I tested (2Xs) when I first brought it inside that tested at dKH = 21.28. I'm confused - what's going on? Could my KH testing fluid be going bad? I've been using it now for a year and it is getting low.

 

If you still have the Nutrifin GH/KH test kit box, look on the bottom. There is an expiry date printed there. It is possible the test is no longer giving accurate results.

KH is sometimes referred to as temporary hardness and is made up of carbonate and bicarbonate anions. Temporary hardness can be removed by boiling water, plant uptake, and through the buffering of acids. It is possible there is something in the water that is affecting the carbonate hardness while the water is just sitting there. Have you tested this water for the presence of ammonia, nitrite, nitrate? It's possible there is even a level of nitrification taking place which can also have an impact on KH.

Edit: I should clarify this a bit because I've made it sound like nitrification could be happening in the pail. Without a home for bacteria, nitrification wouldn't be happening in the pail, but it could be happening in the well or water vein. Acid can accumulate due to the nitrification process and the acid could be having impact on the KH while it's in the pail. While the water is flowing through the water vein to the well, there will be limestone or calcium and magnesium deposits that continuously replenish the KH, but once the water is removed from the well it can no longer be replenished and the acid accumulation can play havoc on the KH.

Testing the water for ammonia, nitrites and nitrates will help us to determine if this is a possible cause, or help to rule it out.

 

Thank you so very much Tootsie! I tested as you suggested and tested the aging water from the aerated bucket. Ammonia = 0, Nitrite = 0, but Nitrate = 5 mg/L (which I believe is the same as 5ppm). I took a new sample from the well and it tested with Ammonia = 0, Nitrite = 0, and Nitrate was showing a bare minimum trace of color. So the nitrate is clearly a bit higher after aging/aeration. I tried to test the KH again on the fresh well water but ran out of reagent (have more ordered). But the GH measures the same 21 drops (dGH = 23.52) as the first well water tested before being aerated so I think I can assume the KH would have tested the same as well. So does that mean there is some bacteria in the well water that is continuing to nitrify in the bucket? If so, shouldn't we have detected some ammonia and nitrite in the fresh (not aged) well water? And if so, does that mean that the chlorine we inject into the water at the well head is not enough to kill all the bacteria in the water? (glad I don't drink the well water anymore!).

Again, thank you so very much for your explanation.

 

No, I don't believe nitrification would still be taking place in the bucket, but if you have a fairly high evaporation rate, the nitrates may be becoming more concentrated as water evaporates and the nitrates get left behind. The same happens with mineral content and can be seen if the water is monitored with a GH (general hardness) test kit. As the water evaporates, it leaves behind the minerals, and the water will test harder than before. Minerals, contaminants and even toxins will become more concentrated as the water evaporates and is one of the many good reasons large frequent water changes should be performed on our aquariums as opposed to just topping off the tank.

A 5 mg/l (for our purposes we can use ppm instead) level of nitrate is well within safe levels and isn't something you need to be concerned about. I'm not sure at what concentration or frequency well water needs to be treated with bleach to keep harmful bacteria from being present, but it would be wrong of us to assume treating the well with bleach would prevent nitrates from being present in your well water. Nitrates can become present due to runoff that contains animal or even human waste. It can enter your water long before it reaches your well. Acids that may be present due to nitrification or other sources, may not all be neutralized on it's path to the well or in the well, and once they have a finite amount of KH to buffer them (in the pail), maybe they exhaust some of the KH resulting in lower KH measurements.

Rain water also has the potential of having acids trapped, and so when you mix the rainwater with water from an other source, the rain water could potentially cause KH to drop more than anticipated as KH is used up buffering any acid present in the rain.

Water is quite a complex substance. Sometimes without more testing equipment than we have on hand for this hobby, it can be difficult to determine exactly why you maybe observing the results you are. The nitrate level in the well water isn't very high so while there is possibly a little nitrification happening in the water supply, I don't think we can blame it for the drop in KH. Possibly acid from other sources are present and are causing this. The best we can likely do, is to observe the behavior of your water as you proceed with this, and figure out how to compensate for the changes you observe to provide you with enough KH to provide a stable pH.

The mix you currently made was by my calculations about a 28% well water to 72% rain water mix. When you get your new KH test, see what effects a 40% well, and 60% rain does for you. Also, lets track the difference in GH these mixes make.