Playground sand raise PH?

[Steve-o]

Hey Guys i have a 55 gallon tank and for about 3 or 4 weeks i have been doing a 15 gallon water change everyday using just straight RO water... the water right from the RO filter has a ph of 6.0... but when i check the ph in the tank 24 hours later the ph will be around 7.6... the GH and KH are fine but the PH keeps increasing... its driving me crazy... do you think its the playground sand increasing the PH??

[ILLUSN]

could be if its co3 based, whats the actual kh and gh of the tank water?

[the german]

when you have silica sand,whats on most playgrounds,you should have no probs.

[Steve-o]

my KH is at 2 (35.8 ppm) and GH is at 1 (17.9 ppm)