Originally Posted by Bigjohnnofish
gee if your rainwater is absorbing that much kh in the form of limestone dust what else does it contain ? sounds like tap water might be better option...
Depends a little bit where you live, the problem for us in the UK is that a huge number of us live on a tiny island, mainly in the South. This impacts on both rain water and tap water quality.
Originally Posted by vespasian
Thanks for the replies. I live in Northumberland and the geology is generally pretty hard (metamorphic granites), so I would imagine the rain water would be fairly soft. I have now done a water change with the rain water and added some tap water to buffer it a little. I will be keeping a close eye on the pH levels.
You may find that your tap water is very low in carbonates, a lot of water in the UK is now soft, but alkaline with a high pH. This is because the water companies are adding NaOH to raise the pH and stop lead (Pb) and copper (Cu) from old pipes going into solution.
This raises pH (pH is a measure of the H+:OH- ratio, and you've added OH- ions), but it doesn't add any buffering, because all the OH- ions are in solution. Have a look at this thread <"http://www.plecoplanet.com/forum/showthread.php?t=9323
You should be able to get water parameters from your water supplier.