Randy Holmes-Farley
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My Tank Thread
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Good question, and I actually don't know. I am curious about it as well, but it had just always been quoted in cycling instructions so I kept it in my suggestions.
There were two or three cases I saw before (on Reddit) where someone was actually measuring ammonia quite diligently and had either one or small clowns, and I calculated the rise in ammonia to be 0.2 to 0.5 ppm a day for them, and this is despite there already be some ammonia oxidation. Of course, that meant I could not really calculate the exact amount of ammonia produced from waste produced by the fish, but multiplying that by the expected amount of fish that should be in the tank and I got between 1 to 1.6 ppm (not including the amount that would be oxidized to nitrite). So I figured yeah, 2ppm sounds like a good number to hit.
IMO, that is likely someone's goal for cycling, not a real expectation of a real tank situation.
In a FO, where there is no ammonia or nitrate consumption, 2 ppm of ammonia added each day means nitrate would accumulate 7.3 ppm per day, which seems on the high side.