I would assume it'll be above their deposit and probably more. 

By NH law, a security deposit remains the property of the tenant and cannot be mingled with the assets of the landlord, so yes, you would need a separate interest-bearing account for security deposit. 

Can Florida landlords make a claim on full security deposit because tenant's actions (specifically unauthorized cat and its pee) caused loss of rent and termination of rental agreement with new renters?

What is an acceptable rent deposit and application fee for section 8 tenants?

do I just say to them there is nothing left for me to return from the deposit? 

For the future, if you get in this situation again (I'm sure you won't), have them sign the lease, give you the deposit and put in writing that should they change their mind before moving in, that the money you get as the deposit is non-refundable.I had someone do that once and I kept the deposit (and they told me to keep it even before we had a chance to say anything).

After searching more, I found a few BP post where people seemed to have one account for all deposits, and keep only deposits in there.

I think your security deposit is too low.

Generally most states want security deposits to be in a separate account (All security deposits in one account but it is only for security deposits) some but not all states expect you to pay interest on the security deposit. 

take double deposit or more if you can and a top of market rental rate.