Koi tank advice

[Dale D]

If you are getting an ammonia reading in a tank with an established filter system then it does not have enough bio filtration capacity.

You either have to add another filter, replace what you have with a larger one, or add more bio media to the existing filter if there is room in it.

To get rid of the Nitrate just do some big water changes.

What is the PH? If it is really low, 5 or so, then you will have to do the water changes slowly so the fish can adjust to the rising PH.