Jump to content

Water hardness test - tap vs tank

Recommended Posts

 Hey AC forum,

New here, setting up a new 30 gallon aquarium and trying to cycle in some live plants and get it nice and stable before adding any fish. 

Situation is: our tap water tested well in the soft range, but after 2 weeks in the tank, the same water we got from the tap is now testing at the high range or “hard”.  The difference is from one extreme to the other. 

We were wondering if the river rocks we added could be adding some mineral content that would cause the hardness level to increase.  

Oops, accidentally hit the send button. 

Has anyone experienced a major difference in water hardness between the tap and the tank? If so what was the cause and how was it corrected?

Adding some pics in case we have any geologists out there 🙂





Link to comment
Share on other sites

I do not have the problem you are describing but I can speculate some of the causes. You can check if your rocks are causing the problem by taking some out and dribbling some white vinegar on them. If it bubbles the rock has calcium carbonate in it and will affect your ph. Looking at your tap water things that can causes changes include:

  • Water main break somewhere 
  • Your water treatment plant could have added more chlorine to the system, usually you recognize this by a stronger chlorine smell
  • Heavy rains
  • Algae blooms if your water supply comes from lakes or rivers, but this will make the water smell fishy. I describe the water as tasting 'green' 

You can call your local water treatment plant and describe your situation and they may be able to provide a reason. PH will fluctuate from day to day, how long has the increase been noted? 

Link to comment
Share on other sites

Thanks! I’ll try the vinegar test. 

I did test our tap water before filling the tank and it was soft (very soft - we live in Everett, WA and if you live in this area, you know we have super soft tap water). That’s the weird thing. After 2 weeks in the tank, the water is now hard. So it kinda makes sense that something in our tank caused it to change.


Link to comment
Share on other sites

Looks like you have some seiryu stone or similar in the tank, which is a limestone based rock. It can definitely increase your ph and water hardness. If you have a stronger acid than vinegar I'd use that instead.

  • Like 1
Link to comment
Share on other sites

On 2/7/2023 at 4:51 PM, Jeff Bogey said:


That rock in the middle there. Those white bands are minerals. That's going to impact water parameters a little bit. Not much, and it will go down over time.

One method some people use is to "prep" seiryu stone by using vinegar on the surface to dissolve out some of those minerals and lower the impact. Same way of soaking wood to remove some of the initial tannins.

Generally your rocks look ok. KH and GH could go up from evaporation as well as from the hard scape.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...