I think i'd need a better look at your gravel to know whether it's contributing to hardness. I'd test water with nothing in it first... then add the gravel, wait a day then test the water again. See if the gravel is doing anything to your water. I know one of my tank which has this large round gravel red that has constantly above neutral pH and hard water no matter how much peat and driftwood I add to the water (hard blackwater..). I soon found out it was the gravel doing it, everything else I have in there is inert or pH lowering (loam pH6, quartz inert). Sorry to say, I don't have any experience with Hagen GeoSystem gravel, so I don't really know what it does.

If he has algae problems... ruslan, limiting light could be a good thing . Ketapang leaf might help lower pH a bit... but won't really help much to reduce hardness. In a tiny tank.. the non pressurized co2 is usually sufficient... in fact it's a bit risky to use non pressurized co2 on a small tank... you can't regulate it.. so can be quite dangerous.

Gary, holland sand where got crushed shells wan? Good quality holland sand relatively inert.