Water Hardness Different In Each Of My Tanks

[Ellieabbott4]

I normally use the API test strips to test my water but I'm now using a nutrafin liquid test kit. I also started to use a jbl kh liquid test kit. I've tested all 3 of my tanks and they all come back at different readings, my 90l comes back at 6 (which is the top end of soft water), my 60l comes back at 4 (which is soft water), and my newly set up 20l comes up at 14 ( which is very hard water). The water coming out the tap is very hard, how could it be so soft in my 90l and 60l?

 

[CSnyder00]

Established tanks can change their own chemistry. They tend to be more acidic. It could also be from the decor or substrate or also from the use or lack of water conditioner, filter media. All of this can affect water chemistry.