nry said:
...It is only 50% weekly on EI to bring the fertiliser ppm down to a low level to 'reset' things and then start again..
dw1305 said:
the "EI method" uses the 50% water change to "re-set" the tank every week, before the new fertilisers are dosed
Once again, for the umpteenth time, it's necessary to clarify that large water changes, particularly those that involve EI dosed tanks, have nothing to do with "re-setting" the tank of fertilizer. This misconception is one of the more egregious warping of the truth, which only serves to generate more myths and illusions.
EI, or PMDD, or any eutrophic dosing system does not really care how often or what percentage water change is performed.
The primary purpose of the water change is to rid the tank of accelerated buildup of organic waste. Organic waste buildup has several negative effects as follows:
1) High organic waste increases the Biochemical Oxygen Demand (BOD).
The BOD can be thought of as the amount of dissolved oxygen required by aerobic (oxygen using) micro-organisms to break down organic waste present in that body of water at give temperature over a unit time period. Therefore, if there is a lot of organic waste present in that body of water, the demand for Oxygen by those micro-organisms is high. Oxygen is a precious commodity in the tank. This should not require an explanation. Plants consume oxygen and they do not produce Oxygen at night. As a result, high levels of organic waste has the effect of micro-organisms and plant competing with fish for Oxygen.
2) Organic waste, gets broken down into it's biological and chemical constituents. One of these constituents is NH3/NH4 (ammonia/ammonium). Another is NO2 (nitrite). Therefore, high levels of organic waste means that high levels of ammonia and nitrite will be produced. This is the same effect as overstocking.
3) Organic waste buildup in a tank has the effect of adding to the thickness of the biofilm by adhering to the plant leaves, thereby blocking gas exchange and impeding nutrient/CO2 uptake by the leaves.
The cumulative effect of these three factors is that fish health can suffer due to hypoxia, anaerobic micro-organism production increase (and their byproducts are unhealthy), fish suffer toxicity due to ammonia/nitrite, plant nutrition suffers due to inefficient nutrient/CO2 uptake and the risk of algal blooms resulting from these condition increases, especially if the lighting is high.
Enriching the tank with CO2, which causes a need for higher nutrient levels, results in higher growth rates and in higher rates of organic waste production. People still haven't figured out that plants produce waste, just like fish do. When you feed your fish more they produce more faeces, CO2 and urine right? Well feeding the plants more also causes them to produce more waste. Depending on the amount of CO2 enrichment there may be 300% to 1000% increase in organic waste production.
Imagine if your fish produce 300% to 1000% increase in faeces and urine. Would you want to do more water changes? The correct answer is YES.
THAT is why EI and other eutrophic dosing schemes elect to do higher levels of water changes. This so-called "re-setting" is just a convenient coincidence that makes the calculations easier. It is also a convenient reality to convince those people, who are paranoid about nutrients, that the levels of nutrients will not rise above a certain level.
But we should not stray from the path. If you feed living things more often then they will grow faster and produce more waste. It's that simple. The problem here is that The Matrix teaches us to fear nutrients and to think of them as dangerous chemicals to be controlled instead of teaching us that they are FOOD.
Therefore as Ed quite rightly pointed out, the amount of water you change should not be ascribed some dogmatic value, but should be considered within the context of how much CO2 and nutrients are being fed and within the context of the factors discussed above, as well as how much time the hobbyist has available for the water change.
Cheers,