We have a large glazed pot, so as already said you will need to use a proper sealant on the inside as well as blocking up the whole - I did this years ago with a friends pot, but I can;t remember what the hole plugging stuff was - it was two plastasine like bits that I squished together and then put it in the whole (sorry not much help there).
We have a water plant in ours and lots of weed for the fish to eat (they don't always get food - even though DS loves feeding them) it gets the sun for a couple of hours int eh afternoon, which is not really ideal - as the more sun the more algae, so if you can keep it in the shade its best.
We don;t use a pump, we used to, not sure why we stopped. Anyway, the fish are thriving, as are the plants and its attracted snails now, so they do a bit of cleaning
Have fun