Sea shell is just calcium carbonate, same stuff in the calcium tabs. So both will raise your hardness and the pH.
The pH is related to hardness. For technical explanation:
pH is measuring the amount of H+ - hydrogen ions - that is a measure of acidity.
The amount of H+ is directly proportionate to the bicarbonate level (that is
KH) <& other buffers>
General hardness (
GH) measures the
Ca++,
Mg++, & other positive ions (but not
Na+ - common salt).
If you add
Ca++, you are also adding negative ions (you must have electrical neutrality in any substance). In seahells &
Ca tabs, the negative ions are carbonates (or bicarbonates), so you are also raising the
KH & so the pH as well.
So, unless you are adding
CaCl2 <Calcium chloride>, you will end up raising the
KH & pH when you add calcium.
BTW, I don't think
CaCl2 will be that great for snails. The shell is
CaCO3 - remember? So they need the carbonates as well. YOu may as well add the seashell & raise both the
KH &
GH (& pH).