Others might know better but I feel ich is just normal flora found in a fish tank like nitrosomas and nitrobacter. It just kinda floats around until it find something to host, in our cases fish that are stressed.
I don't personally believe you can let a tank sit empty for "x" weeks and be free of ich forever.
Ich is very small very distinct round dots, always the same shape and size. A couple of dots during a stressfull time is not worth the added stress of netting and quarantine in my opinion but is a sign to get a quarantine tank running in case the ich gets worse.
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