Hardness

Used a tetra test strip to calculate hardness (will invest in a liquid one this week).

GH - 8, KH - 6. Is my water hard or soft?

Since getting into the hobby this is the one thing that continually gets me a little confused. I've read that GH/KH will make for a stable PH, and my PH never fluctuates from 7.6

Thanks.

It's actually GH that is the soft or hard figure. Strictly speaking GH is permanent hardness and KH is temporary hardness but when fishkeepers talk about hardness they mean GH. It is GH that affects a fish's ability to cope.
Incidentally, the term hard comes from hard water being hard to make soap lather, and the permanent/temporary bit is because KH disappears when heated.

GH is a measure of divalent metal ions, and in the case of tapwater that means mainly calcium, some magnesium and tiny amounts of others.
KH is a measure of the buffering capacity of water composed mainly of carbonate and bicarbonate. Water companies call it alkalinity.
[source of information - son that used to work for a water testing company]

The other term you'll come across in fishkeeping is TDS (total dissolved solids) which measures everything in the water and is a better measure than GH for fish, though a TDS meter is more expensive than GH testers.


Most test kits use German degrees as their measure, some use mg/l CaCO₃, though this doesn't mean the figure is measuring carbonate it's just a unit of what it would be if all the GH was in the form of calcium carbonate. These units do get confusing.

Your GH of 8 looks like German degrees. Looking at my water company's website, they give hardness as mg/l Ca and 8 degrees converts to 57mg/l Ca. The slightly hard band is 40 to 60 mg/l Ca so your water falls at the upper end of slightly hard.
Your KH means you have a reasonable amount of buffering capacity, enough that you shouldn't have any pH problems unless you grossly overstock and don't do any water changes. Nitrate is acidic as are other things excreted by the fish. If the KH is used up, the pH drops. With KH 6, you'd have to really mistreat the tank for that to happen.
Assuming the strip is accurate.

If you do get a liquid tester, they work by adding 1 drop of reagent at a time, shaking the tube after each drop, until the liquid changes colour. The number of drops it took to change gives you the hardness.





Just for anyone else who wants to know, I converted all Northumbrian Water's hardness bands to German degrees.


Soft                       0 - 2.84
Moderately soft     2.84 - 5.68
Slightly hard          5.68 - 8.52
Moderately hard    8.52 - 11.36
Hard                      11.36 - 17.04
Very Hard              0ver 17.04