How can my tap water less hard then my tank water. Help understand this.

If a moderator thinks that this post is better off in a different thread please move.

Hello I am pretty new on this forum but not to the hobby. I never really believed in chasing hardness or pH for my aquariums. I've always had very productive longevity with any aquariums that I set up in my four decades involved in the hobby.

This is my dilemma and I don't understand this need help


I was at my pool store and I asked the guy to test my hardness of my tap water with their fairly new Hi-Tech water scanner.

My tap water was 44ppm which I then converted using 17.8 =1DGH
my tap water was 2.5 DGH (degrees general hardness)

He then tested some of my aquarium water
Which read 106 PPM so using the same conversion formula my tank water was 6.0 DGH

How can my aquarium be harder water then the water I put into it every week. I do an 18 to 25% water change every week. I have a 55 gallon tank so I'm basically replacing 100% of my water every 4 to 5 weeks.

I then used a API test strip.
My GH was 60
My KH was 80

So using the equation of 17.8 is equal to 1 degree of hardness

My GH was = 3.4
My KH was = 4.5


I am very confused how my tap water is softer then my tank water confused! Also my my GH is so off from the two tests the pool store was 6.0 and using API test strips was 3.4 I'm beginning to believe that a lot of these fish products are bulshit snake oil and not that accurate.

there are a lot of substrates, and some Rocks that will raise hardness

Does the scanning device at the pool shop only measure general hardness, or does it measure TDS (total dissolved solids), because they are different?

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Dry paper test strips are not as accurate as liquid test kits. And digital probes/ scanners are generally more accurate than liquid or paper test kits. However, digital probes/ scanners need to be calibrated regularly otherwise they can give incorrect readings.

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If you have calcium based items (shells, limestones, calcium based substrate) in the aquarium, it will increase the GH and KH of the tank water.

Tony blazer said:

How can my aquarium be harder water then the water I put into it every week. I do an 18 to 25% water change every week. I have a 55 gallon tank so I'm basically replacing 100% of my water every 4 to 5 weeks.

Click to expand...

That's not how it works. Water quality deteriorates between water changes, and minerals in the water are used up when they encounter acids in the water. The only exception to this is if there is a calcium based substrate or calcium based items in the tank that will neutralise acids in the water and possibly increase the GH and KH.

If you do a 25% water change each week you leave behind 75% of the bad stuff in the water.
If you do a 50% water change each week you leave behind 50% of the bad stuff in the water.
If you do a 75% water change each week you leave behind 25% of the bad stuff in the water.

The only way you can replace 100% of the water is to drain the tank completely and refill it. Doing small water changes will never dilute the water by 100% because the fish are constantly producing waste that changes the water quality and chemistry.

On the subject of water changes, a 50 or 75% water change and gravel clean each week is better for the fish because it dilutes nutrients and disease organisms more effectively than smaller water changes.

"How can my aquarium be harder water then the water I put into it every week. I do an 18 to 25% water change every week."

18 to 25% is not much of a water change. When you do a water change you remove only about 25% of the minerals in the tank water. Then when you do a water change you add a water conditioner , fertilizer, and other things that add minerals to the water. Then you feed the fish which adds more minerals to the water. Then some water evaporates so you add more mineral Then at the next water change your do another 25 %. So over time the mineral content will go up unless you do a 100% water change once a week. The video below is a demonstration of how effective water changes are using food coloring in a aquarium.


In aquarium with bright light and CO2 you have to fertilize the water more often. In this case a 50% water change is typically recommend.

Additionally as mentioned some rocks and substrates contain minerals that can dissolve in the aquarin. These rocks can cause water harness to increase.

Just a note the GH test only measures the total amount of calcium and magnesium in the water other minerals are not detect by this test. Hard water is typically hard due to calcium and magnesium. Typically hard water has a lot of calcium carbonate and magnesium carbonate but it will could Ca Mg sulfates and chlorides. However in some case the water can have a lot of minerals but very little calcium and magnesium.

The KH test detects only carbonate salts. XCO3 where X is a metal. For example Calcium carbonate, sodium bicarbonate or copper carbonate. Only one of the 3 will be detected by the GH test while all 3 will be detected by theKH test. So the KH test can detect other minerals other than the typical calcium and magnesium carbonates.

There is a third test called TDS (Total dissolved solids) it will detect everything in your water and sand be very useful to manage an aquarium. It is a small electrical probe that look a lot like a PH pen.

CryptFan said:

there are a lot of substrates, and some Rocks that will raise hardness

Click to expand...

Yes I do have lace Rock in my tank and yes

CryptFan said:

there are a lot of substrates, and some Rocks that will raise hardness

Click to expand...

Lace Rock can dramatically raise the pH of aquariums, which is of no concern in saltwater aquariums. In freshwater aquariums, it should only be used with animals (such as African cichlids) that thrive in or are not particularly sensitive to unusually high pH.


Yes I have a significant amount of lace Rock in the tank it's been there for over 20 years and I kept crushed coral in one of my filter bags for years but that has been removed several months ago when I converted the cichlid tank into a community tank.

My tap water pH is b around 8.8 from the tap while my tank water before and after water changes ranges 7.2 -7.4. but this still doesn't explain why my tank water is testing harder then my tap water. It makes no sense to me.

Colin_T said:

Does the scanning device at the pool shop only measure general hardness, or does it measure TDS (total dissolved solids), because they are different?

----------------------
Dry paper test strips are not as accurate as liquid test kits. And digital probes/ scanners are generally more accurate than liquid or paper test kits. However, digital probes/ scanners need to be calibrated regularly otherwise they can give incorrect readings.

----------------------​

If you have calcium based items (shells, limestones, calcium based substrate) in the aquarium, it will increase the GH and KH of the tank water.

----------------------​

That's not how it works. Water quality deteriorates between water changes, and minerals in the water are used up when they encounter acids in the water. The only exception to this is if there is a calcium based substrate or calcium based items in the tank that will neutralise acids in the water and possibly increase the GH and KH.

If you do a 25% water change each week you leave behind 75% of the bad stuff in the water.
If you do a 50% water change each week you leave behind 50% of the bad stuff in the water.
If you do a 75% water change each week you leave behind 25% of the bad stuff in the water.

The only way you can replace 100% of the water is to drain the tank completely and refill it. Doing small water changes will never dilute the water by 100% because the fish are constantly producing waste that changes the water quality and chemistry.

On the subject of water changes, a 50 or 75% water change and gravel clean each week is better for the fish because it dilutes nutrients and disease organisms more effectively than smaller water changes.

Click to expand...

I believe the water scanning device does total hardness.

Totally agree that the dip strips are probably the most inaccurate and the pool water scanning device is probably the most accurate for sure that's why I asked and brought my water for my tank from the test the last time I was at the store he told me about the new device

I'm very comfortable with my maintenance schedule of my tank and the overall quality of my water and the healthiness of my fish and plants. I actually think too much cleaning can be worse for your aquarium. Agree to disagree on that. I do have lace Rock in the tank over 20 years what it was an African cichlid tank perhaps the lace Rock is increasing the hardness. I know that increase hardness of water has a positive correlation to increase PH as KH and DH increase so does usually pH.

My tap water out of the tap is around 8.6-8.8 while my tank is usually sitting around 7.2-7.4

Thank you for the input.

StevenF said:

"How can my aquarium be harder water then the water I put into it every week. I do an 18 to 25% water change every week."

18 to 25% is not much of a water change. When you do a water change you remove only about 25% of the minerals in the tank water. Then when you do a water change you add a water conditioner , fertilizer, and other things that add minerals to the water. Then you feed the fish which adds more minerals to the water. Then some water evaporates so you add more mineral Then at the next water change your do another 25 %. So over time the mineral content will go up unless you do a 100% water change once a week. The video below is a demonstration of how effective water changes are using food coloring in a aquarium.

In aquarium with bright light and CO2 you have to fertilize the water more often. In this case a 50% water change is typically recommend.

Additionally as mentioned some rocks and substrates contain minerals that can dissolve in the aquarin. These rocks can cause water harness to increase.

Just a note the GH test only measures the total amount of calcium and magnesium in the water other minerals are not detect by this test. Hard water is typically hard due to calcium and magnesium. Typically hard water has a lot of calcium carbonate and magnesium carbonate but it will could Ca Mg sulfates and chlorides. However in some case the water can have a lot of minerals but very little calcium and magnesium.

The KH test detects only carbonate salts. XCO3 where X is a metal. For example Calcium carbonate, sodium bicarbonate or copper carbonate. Only one of the 3 will be detected by the GH test while all 3 will be detected by theKH test. So the KH test can detect other minerals other than the typical calcium and magnesium carbonates.

There is a third test called TDS (Total dissolved solids) it will detect everything in your water and sand be very useful to manage an aquarium. It is a small electrical probe that look a lot like a PH pen.

Click to expand...

Thank you for the input and the visual aid I think the pool water scanner is for TDS I will ask next time I'm there. I am very comfortable with my maintenance schedule of my tanks and my water parameters with ammonia nitrites PH and nitrates. I've never really cared about hardness before. I have stated this multiple times I don't really Chase numbers to me it's about consistency with the parameters.

As you can see how fish are kept in most industrial settings not very gentle with water changes and I am so they don't regulate water parameters for every specific species it would be too much time and money for that. Fish are very highly adaptable to a wide range of pH and hardness then what the scientific books indicate. Most fish are from a trees that do not keep their natural environment water parameters. It's most likely related to the 20 lb plus of lace Rock in my aquarium that is over 20 years old. I was just surprised that my drinking water is less the 50PPM and is low on the hardness level. I actually looked it up and is considered to be very good drinking water lol

Is the pH of your tap water that of freshly run tap water or water that has been allowed to stand for 24 hours? It is common for the pH to change on standing so it is 'stood' water pH which should be compared to the tank.
Carbon dioxide dissolved in the water lowers the pH, then it gasses off on standing and the pH rises.
In some areas with low pH, the water provider adds something to temporarily raise the pH to prevent pipework corrosion. After 24 hours the pH has usually fallen again.

If the rocks in your tank are made if calcium carbonate they will slowly dissolve increasing both GH (from the calcium in the rock) and KH (from the carbonate in the rock).

I usually let my water sit for only a half an hour after preparing it in my buckets. I only have a 55 gallon tank so even with a 50% water change it's only five or six buckets no big deal but imagine somebody with a hundred fifty to two hundred gallon tank. I know a lot of people just put tap water into there aquariums directly from the sink and they added their water conditioner before putting the new water in I disagree with this practice but everybody has their own opinions on how to do things.

My tap is 8.8 and my tank PH usually 7.2 to 7.4 before and after my water changes. Sometimes I prepare the water the night before but most of the time about a half an hour.

Let a glass of tap water stand for 24 hours and then test the pH. Compare that to the pH of your tank water.

essjay said:

Let a glass of tap water stand for 24 hours and then test the pH. Compare that to the pH of your tank water.

Click to expand...

I will definitely run a test and post the results but my pH is not my issue in my tank really nothing's really an issue with my water parameters. My fish are healthy and happy.

I was just surprised that my tap water was less hard then my tank water but if my lace Rock which is over 20 years old is releasing extra minerals into my water it makes sense.

I'm going to State this again!! Agree to disagree on this (because I had some issues with some people on this forum)

I have never been a person chasing pH or hardness numbers for my aquariums.

I was taught by my father the focus should be on consistency of the water parameters and keeping low nitrates levels.

Most fish and plants are not raised in their natural habitats and are very adaptable to various water parameters due to the mass production of fish and plants.

This is a very interesting article. Backing up my philosophy that I was taught by my dad back in the seventies.


I'm just glad to know that I have pretty good drinking water less the 50PPM

I wasn't suggesting you chase your pH for the fish's sake, I was just looking at it from the chemistry point of view

Lots of misinformation to sort out in this thread. First, on the questions asked (and already answered by the way).

The GH of the aquarium water is higher than the source (tap) water because there are calcareous substances in the tank and these very slowly dissolve minerals (calcium and magnesium) into the tank water. Lace rock is calcareous. No idea hat the substrate is, but it could be as well. Because you do insufficient water changes, you are not seeing much change in the tank water parameters.

The pH is lower in the tank than in the source water due to the biological processes that occur in every aquarium. There is insufficient buffering though the calcareous items do have some, but here it is not significant. This is not "bad," depending upon fish species.

Tony blazer said:

My fish are healthy and happy.

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There is no way you or any of us can ever know this. Fish behaviour is not indicative unless things are so bad the fish react negatively. All we can do is learn the science and respond accordingly. That is the only guide to healthy and "happy" fish.

You can agree to disagree all you like, that is your prerogative. But you are disagreeing with proven science, so others will call you out because we are concerned that pther inexperienced members do not get the wrong end of the stick, so to speak.

Tony blazer said:

Most fish and plants are not raised in their natural habitats and are very adaptable to various water parameters due to the mass production of fish and plants.
This is a very interesting article. Backing up my philosophy that I was taught by my dad back in the seventies.

Click to expand...

This is a fallacy. The article linked is not scientifically accurate, and they draw misleading conclusions. The water in the fish store tanks has nothing to do with what the fish require to be healthy long-term. Some fish, perhaps most, can usually manage short-term, and fish stores know this and expect to sell their fish quickly. What we provide at home is a very different matter.

Fish do not adapt to vastly differing water parameters in short spaces of time. They do not have that physiological ability. Those who study or have studied biology and ichthyology understand fish physiology and thus know that each species of freshwater fish has evolved over thousands of years to function best in a very specific environment ("environment" including water parameters as well as the other factors). They are not going to change physiologically over a few months or years. They can sometimes evolve over time, sometimes not; this is why we are living through the greatest period of mass extinctions in history (aside from the cataclysmic events like the dinosaur extinction).

The fish's relationship to its aquatic environment is far more involved than the relationship of any terrestrial animal to its air environment. Until we understand and accept this, we are not doing our fish any favours. And they will bee weakened as they are forced to accept inadequate conditions, and they will have shorter lifespans if they do not succumb to something on the way. Stress results from all of this, and that is the direct cause of 95% of fish disease.

As for water changes, as other members have also pointed out, the more substantial the water volume the better for the fish. There is no point in arguing science.

@Byron I find you very condescending arrogant with an elitist attitude. You already was one of the reasons one of my post got closed. Yes it was already established that my 20+ year old lace Rock that I have in my aquarium is most likely the reason that my aquarium water tests higher than my tap water I use to prepare my water.

I never stayed or was it indicated that I had a problem or issue with my water parameters I keep in my aquarium. I have extremely consistent water parameters every week every month. My fish are healthy and thrive in my aquariums and live a very long life span. I will match my tank up and inhabitants with your tanks any day of the week. I never claim of being an expert in the hobby but I have over four decades of experience what about you sir?

Let's talk about science for a second cuz you love using science quotes. In the last four decades how many science beliefs that we had in the 60's 70s 80s 90s ECT have been altered or just plain wrong in there theories or beliefs.

Just like in human evolution. We all started out with black pigment dark kinky hair humans involved and changed over the years to adapt to their new climates environments they migrated to. It's very simple really. Most of fish information that you find can contradict itself about the exact number of pH hardness a fish likes. there is not an exact number there's always a range and you can find different ranges if you look through different scientific books.

The issue I have is the majority of plants and fish that the hobbyist like you and I are exposed to are raised in fish farms plant nurseries. Their environments are not what their native environments are. That will be too costly too time consuming to produce the numbers that are needed. what do we know from evolution. A species will change physical characteristics and environment needs based on the environment they're in. Animals are very highly adaptable and over the decades the majority of fish and plants are reproduced in various environments that are different than their natural environments. That's why I believe people should not be chasing numbers because these numbers are based on native environment numbers and not what the actual environment is in the fisheries and nurseries. Agree to disagree on this because I know you'll never acknowledge that I am correct.

If my water parameters are consistent and my parameters do not fluctuate from week to week or month to month. I don't have issues with water quality ie dirty water ECT how the hell can you say my maintenance routine per week is inadequate just because I don't believe in doing a 50 to 75% water change once a week that is very narrow-minded. Bioload plus good mechanical filtration capacity plus a strong biological filtration setup is the key in my opinion. This tank has been established well since 2002 I have purchased a new filter changed out substrate added different plants and fish species but that tank was never completely broken down in 18 years and my fish always live a long healthy thriving life span.

Like I said I find you condescending arrogant with an elitist attitude towards others that have different opinions and views. can you please share with me how long you been in the hobby and what was the longest fish you ever had live in one of your aquariums. I'm just curious to know what a real expert with all your scientific knowledge . Sorry but this is how I feel about you with the few interactions we have had in my very short one month of being a member here. Sincerely no disrespect intended Tony.

Time in the hobby means little if anything as it is the level of knowledge one accumulates that matters; I research what interests me and I learn from those who really do know. But I have been in the hobby since 1983, so more than 30 years as I had a couple periods in between when I moved and could not maintain tanks and had to give my fish away. I know I presently have fish that I acquired in 2008 (another move) including a Rineloricaria parva that has a 5 to 8 year normal lifespan and is now in its 11th year. My male Mikrogeophagus altispinosus was in his 9th year when he died, pretty good I would say for a fish with a normal life expectancy of 4 or 5 years.

Considering you have ignored or ridiculed every other member's advice as well as mine, I would not be quick blaming me. And the bottom line is, the science is there and I don't give personal opinions.