Hi SF,
Whilst I am very aware of pathogenic organisms that inhabit the water column, to which non-pathogenic organisms are you referring? Partly my reason for asking is that I have been considering the purchase of a UV-C sterilizer to kill off pathogens but I wouldn't want this to harm non-pathogenic organisms.
JPC
Right now I am talking from the top of my head but you can read on biofloc technology studies for more details. Biofloc technoloy is a type of non-water changed aquaculture system that tries to achieve nature in a closed system.
Non-pathogens could be pretty much anything, certain protozoa, nematodes, other bacteria, etc..From all those there are some dangerous to fish and some harmless.
I wouldn't worry about killing non-pathogenic organisms in the water column because the ones that matter inhabit the filter/substrate, according to studies. The assembly of micro-organisms in the water column apparently changes throughout the day and can be influenced by many factors, as simple as what type of food one fed the fish today. In fact, feeding certain foods apparently can enhance the prevalence of certain organisms.
On another hand,they say the assembly of micro-organisms in the filter is fairly constant.
Since none of us owns the technology to test our tank water on a daily basis for the type of micro-organisms that are prevailing in certain scenarios, and science is still not advanced enough on the topic, it is best to keep the water clean, via water changes being the easiest but I suppose a UV sterilizer can also help. I personally think it's too much trouble as it doesn't guarantee it would be effective against all type of pathogens where a large water change would certainly reduce the amount of anything floating in the water column to a minimum, plus reduce other toxins. A tank is never sterile,and can never be but it is possible to keep the water fairly clean, enough so fish's immune system can cope with any minor stuff.
I think the key of understanding how to achieve nature in aquariums is studying those micro-organism and learn how to influence them to our advantage. So far science has been very poor. We have only recently made some leaps on the type of nitrifiers that get established in aquariums after false beliefs for decades(i.e. archaea) I personally believe that one day it will be possible to purposely set up and "maintain" an aquarium without water changes but right now without advanced knowledge available, if one strikes that balance, it is like winning the lottery. Whoever figures it out fully and is able to reproduce that system again and again, achieving healthy and vibrant fish, can get rich....All fish farms are trying to get there right now as the dangers of the usage of antibiotics on fish for human consumption is being highlighted and water changes on a large scale is not just expensive, it is dangerous, as one can pass on aquaculture microbes to the environment.
