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How often and how much % of the water should I change when I change the water? I have a 20g cycled and filtered sorority (8 girls, 6 neon tetras and 3 Albino cories) Thank you for your help!
 

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about 20-30% weekly would be fine, once you are totally sure the tank is fully cycled. You can change up to 50% safely but with something that big I would go closer to half that because it's a lot to change and you shouldn't have problems keeping the good bacteria established.

17 fish in a 20g seems like a lot to me but someone else will have to comment on that as I haven't set up such stuffed tanks before.
 

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In a 20gal filtered, cycled, fully stocked without live active growing plants....50% weekly with substrate vacuum in all places that can be reached without moving anything....if you have live rooted plants that are thriving-avoid the roots and only do light vacuuming-but I would still do 50% if you have regular gravel or sand substrate.....

Filter media needs a swish/rinse in the bucket of old tank water with a water change a couple of times a month to maintain good water flow.....

The reason I always recommend 50%-especially in heavy stocked tanks-is for the DOC's (dissolved organic compounds) that we don't test for that can be harmful to the livestock and then the nitrate that can build up that can be harmful at high levels long term-(best to keep nitrate under 20ppm)

Since the good bacteria are sticky and adhere to all the surface areas, in the top layer of substrate and in the filter media-very little are in the water column itself and so water only changes will not hurt the nitrogen cycle-but over cleaning can...remembering that the good bacteria can be suffocated if too much mulm/debris accumulate on the substrate and in the filter media-limiting oxygen and food for it to colonize-the weekly vacuuming in non-planted tanks should prevent that and the couple of times a month cleaning of the filter media...but removing and replacing all the water as often as you want shouldn't causing any issues, however, its still a good idea to monitor the water prams regardless.....

Love to see some pics of your tank.....
 
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