Seachem Onyx and water hardness problems!

[ericbullock]

Knowing what I know now I would definitely have made different choices when setting up this little 32 gallon tank. There are three bags of Seachem Onyx in there, on top of a layer of sand. For the life of me I cannot seem to get my water hardness down where I'd like it. Right now the total hardness is around 9.8˚! I've even invested in a RO/DI system and all my top-off and water change water is effectively at 0˚KH. After a 50% water change the total hardness might drop down to around 7˚KH, but then it creeps right back up. With such hard water it is also quite difficult to get my PH down to where I'd like it. Right now it hovers between 6.7 and 7.0, and I'm injecting CO2 like mad!

So....this weekend I've extracted roughly one bag of the Onyx and replaced it with Flourite. I'm tempted to remove all of the Onyx, but given the (now) Onyx/Flourite/Sand mixture I think I'd be hard pressed to get it all separated. This stuff is too expensive to discard! So I have some questions:

1) Is it possible that the Onyx is really buffering the water this much or is something else happening?

2)Does anybody have any little tricks for separating different substrates?

I would remove all the Onyx if there was a good way to do so, as much as I would hate to wreck an established planting of stargrass.

[Ghazanfar Ghori]

The only thing you could probably do to seperate the substrate out is to
use a mesh of an appropriate size and seive it apart.