What in the tank makes water harder?

maybe a rock or the gravel.

want to make it harder

I had my water tested at a store, and it came up to about 0 KH. wassup with that? how is that possible? The PH is fine, so it's not a big problem. Does the hardness itself affect fish?

how can I change it to be harder? Some crushed coral? calcium carbonate of some sort?

-Josh

The "degrees of KH" scale measures the water's alkalinity, its ability to withstand small additions of acid without the pH dropping. Its "buffer." Phosphates contribute to the "buffer"-- you might even be adding them as your "pH buffer"-- and the KH test measures any PO4 as if it were carbonates.

You can "stiffen" the resistance of your water to acids by adding a little slowly-dissolving carbonate to the system. Many of us with supersoft (wonderful!) water like yours, buffer the system with a little cushed coral in the filtration. A little sodium bicarbonate will dissolve all at once, giving a big boost to the pH, but with nothing in reserve to help stabilize it.

I don't know whether your tanks are planted, but there's no reason to increase the KH beyond what will modestly stabilize the pH. Cleaning the filter, etc. keeps those organic acids from overwhelming the light "buffer" of your "soft" water. If you were keeping specialized fishes that have adapted to highly alkaline (highly "buffered high pH) waters, like Rift Lake cichlids, or mollies, etc) then you'd have a special situation and you'd want more dissolved salts including carbonates.

Otherwise...

Don't worry! Be happy!

Crushed coral & shells, limestone (depending on region), ancient sea rocks, etc. all contain high amounts of calcium that will make your water harder. Many fish require hard water, like Salfin Mollies and various cichlids, so its good idea to look up profiles of your fish to see what their needs are.