I have recently moved into a place that has a concrete pond. The pond wasn't maintained the best by the previous tenants or even at all. There are some cracks in it now. And i honestly have no clue what to use to fill them or if i need to use some kind of sealer over the concrete too. Can some one help me out or point me in the right direction please

You could buy some pond underlay and liner, and simple line the concrete area with this as you would normally with a pond, as long as the structure is still stable to hold the water though, I am not sure if it's okay to leave cracks in concrete though? You could always fill them in and put the pond liner over the top.

If you were to just fill it with concrete or other products, then yes these would have to be sealed, as they may leach things into the water.

Where are they cheap from? lol