My pond test strips I bought on Ebay finally arrived after waiting 19 days for Royal Snail Mail to deliver them, so I did a test. Most things are OK, (NO3=10, NO2=1, a bit high, KH=6d, ph=between 7.6 and8) but the GH is very high at 16d. The instructions with the test strips say to do a part water change using rainwater. As it is not very rainy at the moment, is there another, safe, way to lower the GH a bit?