11-21-2009, 12:03 PM
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I think TM is correct, white gravel is often calcareous, comprised of dolomite, marble, or crushed coral. It is primarily intended for marine, rift lake cichlid and livebearer tanks, fish that require hardness and higher (alkaline) pH conditions.
I would recommend replacing it with a darker inert gravel, regular aquarium gravel or similar. Buying it in bulk from the fish store will be less expensive.
On the partial water change, twice a week is fine but it should be more than 20%. I would recommend 40-50% twice a week, or 70% once a week. Larger cichlids produce a lot of waste as you know, and it is the urine and liquefied solid waste that can only be removed by a water change. Filters and bacteria break down the solid into liquid, but they can't remove it. However, change your substrate first, then go to the increased pwc. I think you will see the improvement in your fish.