Hi Chris,
I am not sure whether I understood your question correctly. Maybe this is doing what you want:
ClearAll["Global`*"];
dspecies = {Cl, Ca, Na};
Evaluate[BC[#] & /@ dspecies] = Thread[(Derivative[1, 0][#][L, t] & /@ dspecies) == 0]
Check the result with:
?BC
Regards -- Henrik
