The biggest problem when it comes to ich at chain stores like Petsmart & Petco is the way they filter their tanks. Big chain stores like that run row tanks that are run off a central filtration system and the water flows through all the tanks and only one big giant filter is run in a side room for all the tanks. So one fish in any of those tanks has ich and that means all the fish are subjected to it. That's the main reason I won't buy fish from any store that uses a central filtration system. If each tank in a shop doesn't have its own filter then I'm not even considering buying a fish there.
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