Why is my pH so low? I heard Onyx Sand has inherent buffering properties
which would help to counteract the falling pH.


If you're injecting CO2, you have to expect the pH to get lower. Even if
you're using Onyx. Increasing KH (which Onyx does) won't reduce the pH swing.
Read about it he

www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

Judging from your KH and pH values, you may have too much CO2 in your tank.
(Your tank is small, so this is a possibility, even with DIY.) If there are
fish in the tank, you should increase aeration immediately, to drive off some
of the CO2. Try to keep your CO2 in the "green" range of the chart (see above
URL). You can do this by increasing or decreasing aeration, and/or increasing
or decreasing the size of the yeast/sugar batch.


Leigh

http://www.fortunecity.com/lavender/halloween/881/