Ich, Ichthyophthirius multifilliis, isn't necessarily brought on by ammonia spikes alone. As you seem to know, it's a parasite and inhabits tanks even when not noticable. When certain situations arise, low water quality, stress, etc. the parasite shows itself, and as you said, spreads like a plague.
A simple and natural way to treat the problem is with salt and raising the temperature in your tank, my preferred method.
TREATMENT: To do this, you need to raise the temperature to about 80 degrees farenheit. In addition "It is very easy to treat Ich with salt. You’ll need a concentration of 0.3% salt (3 teaspoons per gallon) to eradicate the Ich. To reach 0.3% salt you would add 1 teaspoon of salt per gallon of water to your tank 3 times, each 12 hours apart. Predissolve the salt in tank water and then add slowly to a high water flow area.
It is important to keep track of the salt going into your tank because salt does not evaporate and is not removed with activated carbon. It is removed only with water changes. To keep the salt at the desired level, when you do a partial water change, you must salt the new water to the same concentration as the tank water. For example, if you remove 10 gallons of water you will need to add 30 teaspoons of salt to the change water to keep the tank at 0.3%." Remember, this is aquarium salt, not table salt.
Ich has a lifespan of about 5 days. I suggest treating with salt for a week or so, when no more visible signs of ich are there.
This should clear the ich up. If not, there are a number of effective, but less preferred chemicals that should work. Quick Cure, for example, has worked for me in the past.
Good luck, Bill
