12 hours is fine. If you have any algae issues etc. it's because you have poor water quality. If you are still cycling, you will see a diatom outbreak and then an algae bloom, which is completely natural.
By the way, hermit crabs and snails do not eat silicate (glass) shells of the diatoms. The diatoms will naturally loose their source of silicate in the water and go away. If you used a silica based sand for your substrate, you will ALWAYS have bad diatoms.
I hope this helps.