Can you please help me with:

I deposited R20000 in a unit trust at a bank 2 months ago. On receiving my statement I saw that i received R12.33 odd rand as interest but that the Unit trust balance acturally decreased to R19715.71 as at 30 June 2011. How do I account for that in my books. When I made the deposit, my bank account obviously was debited with the R20000 and I opened a Bank Account for the Unit Trust in my books. That is the only transaction that I entered.
Now I am not sure how to enter the decrease. I think that the 12.33 needs to go to an interest income account but the decrease in the unit trust I am not sure of? Can you please try and help me. Did I initially enter the R20 000 correctly?