For me.....testing for ammonia and nitrite are needed only when starting a new tank, once the tank is established I barely ever test for those parameters,
After becoming established, or if adding stock, .I use regular pH and nitrate testing to determine frequency and amount of water changes it needs.
Because I want nitrate levels to be as close to zero as possible, and pH to be as stable as possible, regular testing until a proper water change routine is developed lets me know what that water change routine should be.
If I do a water change on Monday showing nitrate at 5ppm(or less), and 8.2 pH, but if either pH has dropped significantly by Fri, or nitrate has risen to 20 ppm by Fri, that tells me (with my stocking level, and my tap waters buffering capacity) that I need to do at least 1 or more water changes between Mon and Fri.
For me, when my 300 gal system in stocked with 15 medium size fish, and a cleanup crew of shrimp in the sump, and heavy planting, regular testing results over time has showed me how to maintain my undetectable to 5 ppm nitrate goal, (and a rock solid pH of 8.2).
That I need to do between, 30% to 40% changes of the systems water volume every other day,
Because each tank is its own entity, when starting a new system, I usually keep a testing log for up to year, to watch trend, and determine that tanks individual needs, and stability.
And/or if a drastic fluctuation in extra stock, is done, or a revamp of filtration is realized, I might do a barage of tests again to reflect those changes.

After becoming established, or if adding stock, .I use regular pH and nitrate testing to determine frequency and amount of water changes it needs.
Because I want nitrate levels to be as close to zero as possible, and pH to be as stable as possible, regular testing until a proper water change routine is developed lets me know what that water change routine should be.
If I do a water change on Monday showing nitrate at 5ppm(or less), and 8.2 pH, but if either pH has dropped significantly by Fri, or nitrate has risen to 20 ppm by Fri, that tells me (with my stocking level, and my tap waters buffering capacity) that I need to do at least 1 or more water changes between Mon and Fri.
For me, when my 300 gal system in stocked with 15 medium size fish, and a cleanup crew of shrimp in the sump, and heavy planting, regular testing results over time has showed me how to maintain my undetectable to 5 ppm nitrate goal, (and a rock solid pH of 8.2).
That I need to do between, 30% to 40% changes of the systems water volume every other day,
Because each tank is its own entity, when starting a new system, I usually keep a testing log for up to year, to watch trend, and determine that tanks individual needs, and stability.
And/or if a drastic fluctuation in extra stock, is done, or a revamp of filtration is realized, I might do a barage of tests again to reflect those changes.

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