11-30-2011, 04:38 PM
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Everything must be kept in perspective.
Many fish carry ich though spots may not ever be seen. Ich first attacks the gills--which is why you see the fish "flashing," which is the term we use when the fish will scrape itself in the gill region against a leaf, wood, rock or sometimes the substrate in an attempt to dislodge the parasite. Fish build up an immunity by producing more natural slime which the parasite cannot get through normally, but the gills remain vulnerable. However, if the fish is weakened by stress, the parasite has a better chance. New fish from stores are obviously highly stressed, and if they are carrying ich it will usually increase.
Any stress-inducing factor can cause an outbreak of ich. And a sudden and significant drop in temperature is one of these. But it has to be pretty severe. If the heater failed during a winter night for instance, and the tank temp lowered from say 77 to 65F, ich would likely appear. If just one parasite is in the tank (due to the gill issue above), that is all it takes because the sudden temp decrease has weakened the fish. But a relatively minor fluctuation will not do this. I deliberately lower the water temp during a water change, sometimes up to 4 or 5 F degrees to stimulate the fish. The store is grasping on a part-truth as an excuse, or perhaps just not knowing better, to give them the benefit of the doubt.
It is more likely that any rise in ammonia or nitrite during a cycle will cause an ich outbreak in new fish, again because the toxicity weakens the fish and they are more susceptible. I have recently had a couple fairly minor outbreaks with new fish, solely due to the stress of netting/bagging/transport/new environment. This is where a QT is useful; isolating the fish for a couple weeks (it can take more than a week to appear as visible spots) allows quick treatment without messing up the main tank. I must confess I do not always QT. But I also don't always have ich with or without QT.