Different PH Levels

[capper]

Our 16 gallon tank has always has a very consistent PH level of about 7.7. We have our new 48 gallon tank and it has a PH level of 6.5 or so. The tap water here in Tokyo seems to be the same. So what could account for the difference? My guess -- could it be that in the 16 gallon tank we are using stones / gravel while in the 48 gallon tank we're using soil stones?

Any ideas? Should we be content with the PH we have or try to alter it?

The big tank has 5 african lampeyes and 3 platties. The smaller tank has 12 cardinal tetras, 4 corydoras, 6 glowlight tetras, 1 guppy, and 5 rummy nose tetras.

 

[Lotus]

It's almost certainly the substrate that's changing the pH. I'm not clear on what the tapwater pH is there; is it 6.5 or 7.7? You should test the tapwater after it has sat out in a cup for 24 hours to get the true pH.

 

[TLH]

Soil stones?I'm not sure what exactly they are but soil here contains Humic Acid like Peat does,although not to the same extent,which would soften the water somewhat.