How To Prevent Ph Swing?

[mwood2]

Well my 20g tank I'm working with has a substrate which lowered my PH to around 6.2-6.4 where as my normal PH from tap is 8.0, the tank is currently fishless so I'm not worried yet but if I do a 50% water change and the water is 8.0 added to 6.2 wont that cause a massive PH swing and if so how do I avoid this?

 

[KittyKat]

Once the fish are in, just don't do such large water changes.. for this water change, you don't have to worry if there are no animals in there. The pH should drop back to its usual within a day or so.