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Phytoplex is "okay" but you would honestly do better with live cultures or if available LiquidLife's Bioplankton
. Feeding more than twice a week in such a small tank is also not really neccessary and too much will add to unwanted algae growths.
The hippopus typically does best on the substrate but placement greatly depends on it's coloration. Brown usually means it will be fine on the substrate given you have PC
or other higher intensity lighting. If it has a decent amount of color, then the zooxanthellae will need supporting much more and the clam may need higher placement depending on lighting.
For a 10 gal
tank, supplimentation over and above regular weekly water changes probabley will not be needed. If that does not keep up with the CaCO3
demand, a two part liqiud ionic compound would work best. Always remember if you do not test, do not add.