When I started mine I used a dechlor every time I topped off the pond (every 3-4 days) and after a while I weighed the cost and decided on a charcoal filter. I got mine at a camping supply, and it's the same one they use for RV's to keep drinking water free of chemicals. I saw one on a pond supply store and went to a local RV place to get it.
Since then, the only thing I put in the pond every year, a couple times a year, is Algaefix.
I attached the filter to the spigot and connect my hose to it for filling the pond and also for rinsing out the brushes. It lasts me over a year....I tested the water the other day and the chlorine/chloramines are still very low.