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Mike S, I was following the flow of topic on this thread and I missed your post asking to clear up your confusion about ther terms 'closed system' and open system'. Sorry.
A closed system would be one that depends on the parameters of recirculated water for the survival of the inhabitants. This requires the harnessing of biological principals to reduce or slow down water deterioration and toxicity build up ,which is the normal consequence living inhabitants have on their environment. Because closed systems are not open ended in their potential to maintain fish life indefinitely they must be periodically 'readjusted' to a base line of non toxicity and 'rechanged' with those elements that have been exhausted.
An open ended system is one that may appear physically closed but actually depends to a large extent on a constant dilution fact to keep the consequences of a true closed system from happening. Generally speaking, these are easier managed systems as you are not depending on 'closed end' support apparatus. pH crash for instance would be an impossibilty in open systems. Because you are not depending on the biological filter to maintain a base line ,the burden of water quality falls equally or solely on the constant dilution factor provided by inflowing water. It is indeed possible in complete open ended systems not to have nitrification going on to any extent at all in a biofilter. Perhaps the most basic way to say this is that a biofilter in an open system is NOT in equilibrium with the population as the true nutrient source is diluted and there for the filter is supporting an 'illusionary' population.
The ultimate experiment that demonstrates open systems is the famous study done on trout held in tiny glass containers that were connected to a constant inflow of water and food. The fish grew like 'ships in a bottle' until there was very litte space left for them to even move. Cruel but it makes the point. If the water were changed every other day for these captives and stopped inbetween the experiment would have been over on day two- the subjects would be in fish heaven on the second day!
The degree of 'open-ness' of a system can be debated. What you are doing is having the semi-open nature of your systems adjusted to more or less "filter dependent/ open system supported" based on feed back readings of TDS.
JR
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