What does 'in line' actually mean?
How do you define 'maintained'?
How are you sure the ecosystem is not affected by larger water changes?
If two identically stocked heavily planted tanks are run side by side, one being low light and no ferts and receives 10% water changes once a fortnight and the other is high tech EI with regular glut dosing and injected co2 and receives one 50% water changes every week, which tank is more stable?
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In line would mean acceptable nitrates and ph,not readings from a tank that has been neglected for to long(going to ask me how long is too long?)
Too long would IMO(all of this is IMO ) be resulting in ;high nitrate readings(what is high?),unhealthy fish,and a pH that is not close to how the source water adjusted to natural after outgassing.
I define maintained as water changed to keep pH accurate and nitrate at an acceptable limit.
Also rinsing /cleaning filters and mechanicals along with any chemical filtration that may need to be replaced or removed.
What is acceptable I suppose is next?
All fish have different ranges and what you are doing with your fish may play a role also.
I don't think most keepers need their tanks as "clean " as my GBR breeding tanks and fry/ juvi growouts?
How do I know large waterchanges don't effect the "eco system"?
I think they do!
In a positive way that is.
I test and have tested for years(and years and years).
Every tank is different but once you can understand the pattern your tank has then you can try any number of mantainance plans and see which gives the results or gets you closest to the results you are looking for.
The point was a "neglected " tank and fish may have volumes of nutrients ,DOC and a pH that was not how the tanks water was when installed.
To do a large water change on such a tank would be stressful on the fish IMO and smaller changes to get the water and fish back to where the source water is the best approach.
Hope I answered OK and this really helps?
The 50% mandatory on EI is because the ferts are dosed in excess and need to be "zeroed" out so the whole process can start again.