But that is exactly what I am saying... UV lights, anything that only affects a part of the tank will never, ever get rid of 100%. It can come darn close, but never 100%. The curve is an asymptotic function that gets very close to 0, but will never be exactly zero given infinite time.
Without getting into the differential equations that govern the system, let me illustrate an example:
Lets start with a 100 gal tank, with a concentration of ich as 100 ich organisms per gallon. Further, lets assume that the UV flows 1 gallon per minute, and 95% of all ich that goes through the light is killed. Further still, let the return from the UV steralizer be instantly mixed with the entire tank (that is the tank is perfectly mixed, no ich hiding in the corner). Lastly, lets say the reproduction rate of ich is in one hour, on average, 10 ich organism spawn into 11. So the rate of ich production is 1 ich per 10 existing organisms per hour or (1/60) of ich per 10 existing organisms per minute.
(Please note I made all those numbers up on the spot, to do this correctly the real numbers would have to be looked up)
* At time 0 (starting time) there are 100*100 = 10,000 ich total
* In one minute 1 gallon goes through the UV. The 1 gallon contains 100 ich. 95% are killed in the light, therefore a loss of 95 occurs. But, the ick in the tank are still growing. (1/60)*(1/10)*10,000 = 16.66 gain
The net gain/loss is 95-16.67, so the average number of ich after one minute is 10000-(95-16.67) = 9921.67. So at time 1, total number of ich is 9921.67
* In the next minute another gallon goes through the UV, but 100 ich don't goes through, only 99.2167 since the concentration went down. 95% of these die: 94.256 die. (1/60)*(1/10)*9921.67 = 16.536 spawn.
Net result is that there is 9921.67-(94.26-16.54)=9843.95 ich left.
Look at what happened. In the first minute the UV light netted a kill 78.33. In the second minute is only netted 77.72. It will go down every minute. Until the kill rate of the light is the same as the reproduction rate of the ich.
* after 1 hour, the average net kill is only 48.87
* after 10 hours, the average net kill is only 0.70
This ananysis works until a very small number of ich organisms are left, then the random nature of the process takes over. But, basically I wanted to show you that there is always an ich that will survive and not go through the UV light. That is why medications need to be used as they treat the entire tank.
Note that the numbers above are only averages, since there are a great deal of random things that occur that can change this analysis.
Real life is even worse, since tanks in no way are perfectly mixed. Using solely a UV light, an ich can hide behind a plant or in the gravel and never get mixed into the water column and therfore never go though the UV light.
This same anaysis can be used for ammonia levels. The major difference is that ammonia is not self-propagating. That is 10 units of ammonia do not become 11 spontaneously. Ich reproduces. The rate of generation of ammonia is fixed given a certian fish fed a certain diet at a certain temperature. The rate of ich is not fixed, as the population continuously grows.
So long as your filter has enough bacteria in it, the rate of ammonia generation by your fish can be the same as the rate of ammonia break down in your filter. That can settle to zero. But, since ich reproduces, the amount of ich after one hour is based on the amount if ich that was there to start with.
Mathematically: Rate of ammonia production = a constant
Rate of ich production = current concentration of ich to a power (like squared)
This is why filters work for reducing the levels of ammonia to zero, even though they only treat a small volume of the tank, while UV lights do not reduce the ich to zero. Finally, at such a small concetration of ich, the fish may be carriers, but non-symptomatic. That is, if there is only on small ich in their gills, they may not be showing the irritation that a heavily infested fish would.
