When you just top off the water level, you are allowing the nitrates and ammonias to become more concentrated,... both the nitrate and ammonias are caused by your fish peeing, pooing, and from uneatten food that sinks to the bottom and decomposes........ (yes, fish do pee
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In nature there are very few natural ponds and and lakes that don't have a source of fresh water flowing into them or out from them,.......yes, filtering your pond does remove some of the more solid wastes, but not all of the liquid based ones......
As for high levels of nitrates and ammonias, they are the main cause behind fin rot, fungal infections and ick, among other things........
With all the rain we've been having, my pond seems to be overflowing at least once a week, basicly rain water in pond water out.......so I haven't been doing any water changes in mine this year.
As for water changes, I can say this,.... I maintain a heavily planted 100 gal aquarium with assorted tropical fish and a 75 gal brackish water tank with 3 g.tiles (fresh water moray eels), I try to do at least a 25% if not 50% water change at least once a month,..... These tanks are just too big to break down and clean like you would a 10 gallon aquarium. By doing these water changes it allows me to vaccume wastes from the gravel and lower the nitrate and ammonia levels at the same time, and...... the 100 gal has been set up for two years now without any problems......
vicky