-pinky-

I don't mean to sound stupid, but when people say soft or hard water, what do they mean? Is it do with the level of pH?

Dozey

No stupid questions here... We all learn from one another.

Hard water is simply water that has high mineral content. Usually consiting of calcium , and magnesium metal cations, and sometimes other dissolved compounds such as bicarbonates and sulfates.

pepetj

Yes it has something to do with pH. As a rule of thumb, soft water and low range pH go together, as well as hard water and high range pH.

In other words, acidic tanks are possible with soft water (e.g. most Discus tanks), alkaline tanks are possible with hard water (e.g. most African Cichlids tanks). Also brackish tanks have higher pH and hardness values than freshwater, and saltwater even higher than the previous two.

Not a stupid question at all.

Pepe
Santo Domingo

Red1313

These two usually go togeather so I thought that this might answer you're question.
From my Nutrafin test kit:
(GH) General Hardness- measure of all the dissolved salts in the water. These salts primarily consist of calcium (Ca), and magnesium (Mg). GH also refered to as Total Hardness (TH).
Why test GH?
the concentration of dissolved salts affects the osmotic regulation systems in fish (internal vs external salt concentrations). GH also influences Ca levels in the blood. Before adding new fish you should test the GH and attempt to match it to the species original habitat. (use peat to soften or limestone to harden)
(KH) Carbonate Hardness- Often refered to as alkalinity is a measure of the water's ability to neutralize an acid (buffering capasity). In the std. aquarium situations with a pH between 6-8.2 alkalinity is composed primarily of bicarbonate ions (HCO3-) and cabonate ions (CO3-).
Why test KH?
KH stabilizes water pH. It is an important source of energy for nitrifying bacteria. Carbonates are also used by plants for photosynthesis when C02 is absent.

So in summery GH (the hard or soft that you refered to) affects the osmotic presures that the fish will have to deal with. For example cichlids that require hard water are designed to pump salts out of their body while fish from soft waters are designed to hold salts to prevent them from being pulled out into the water. If you put a soft water fish in extreme hard water then they will be unable to get rid of the extra salt in they're body. (Think of a freashwater fish in a saltwater tank or viceversa) .
KH is how stable you're pH will be. I have really high KH values in my water so I would have to add a lot of acid to the water before my pH would change. If you have a low KH then you'll have a constantly shifting pH.
Hope this helps.

P.S. High KH usually = High pH, Low Kh usually = low pH