Can i use a 20 gallon filter in a 21 gallon fish tank

But please, whatever you do, get a filter for your aquarium. Your fish will love you for it.
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Hallo from Brussels.
I have a pond (5m/5m/1,5m) with KOI in my garden (in the middle of the city) with filter and pump. It came with the house when I bought it. I never had any experience with ponds or fish. They are doing well (10 years)
However, I love your approach and want to try it. First adding lots of plants and then gradually diminishing the pumping and filtring.
The bottom slopes from a depth of 5 cm to 1,5m. There is ample room, rocks and pebbles to have many more plants than I do.
Two questions:
1. can’t you really tell something more about the kinds of plants that are most efficient for nitrates etc…. I understand it depends on the climate, but not even something about the genera?
2: my KOI eat the waterlilies. alternative to provide shade in summer?
Are these filters just for fish, or will they work for aquatic turtles as well?
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My mother would fashion the mixture into balls about two inches in diameter—two to three pounds of fish would allow a dozen or more substantial fish balls—and then poach these gently with a few slices of carrot. As the gefilte fish cooled, a jelly of an extraordinarily delicate sort coalesced, and, as a child, I had a passion for the fish balls and their rich jelly, along with the obligatory khreyn (Yiddish for horseradish). XP-09 Ultra Silence FishTank Filter External Aquarium Filter Fish Tank Water Pump Remove Oil
Photo provided by FlickrPls suggest me best filter for my 500 Ltr fish tank
Photo provided by FlickrDualband filter set ideal for simultaneous viewing of commercially available Aqua and Gold FISH probes.
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Aquarium filtration removes contaminants, and provides a constant flow of water to aid in fish respiration by dissolving oxygen in the water, and removing carbon dioxide. Proper filtration is critical for maintaining a healthy aquatic environment.

An aquarium filter performs three types of filtration; Mechanical, Chemical and Biological. Learn more about filtration, great products to help you keep crystal clear water and healthy fish by visiting our Easy Care Guide step Water Problem Solving.Pinning this to save and show my brother. He breeds fish and sells them. Has tons of tanks. How To Make: DIY Canister Filter (Aquarium Filter) - YouTubeThe purpose of the filter on your aquarium is to remove , decaying organic matter,free-floating particulate, dangerous chemicals, and the fish's wasteproducts from the water.Our gefilte fish was basically carp, to which pike, whitefish, and sometimes perch or mullet would be added. (The fishmonger delivered the fish alive, swimming in a pail of water.) The fish had to be skinned, boned, and fed into a grinder—we had a massive metal grinder attached to the kitchen table, and my mother would sometimes let me turn the handle. She would then mix the ground fish with raw eggs, matzo meal, and pepper and sugar. (Litvak gefilte fish, I was told, used more pepper, which is how she made it—my father was a Litvak, born in Lithuania.)Gefilte fish is not an everyday dish; it is to be eaten mainly on the Jewish Sabbath in Orthodox households, when cooking is not allowed. When I was growing up, my mother would take off from her surgical duties early on Friday afternoon and devote her time, before the coming of Shabbat, to preparing gefilte fish and other Sabbath dishes.When she arrived each Thursday morning, we would set out for the Bronx to do some shopping together, our first stop being a fish shop on Lydig Avenue run by two Sicilian brothers who were as like as twins. The fishmongers were happy to give us carp, whitefish, and pike, but I had no idea how Helen, African-American, a good, churchgoing Christian, would manage with making such a Jewish delicacy. But her powers of improvisation were formidable, and she made magnificent gefilte fish (she called it “filter fish”), which, I had to acknowledge, was as good as my mother’s. Helen refined her filter fish each time she made it, and my friends and neighbors got a taste for it, too. So did Helen’s church friends; I loved to think of her fellow-Baptists gorging on gefilte fish at their church socials.