Put a nylon stocking of peat moss in your filter. Small tank, a tablespoon or two. Medium size tank 1/4-1/2 cup. Larger tank a cup or more.
When I need to make water ahead of time for water changes I will fill up a knee-hi stocking with peat and put it in a 32 gallon garbage can of water, and run a small pump overnight. The peat can be reused several times, but it needs to steep longer as it wears out.