I have a screw on my gate valve on my skimmer, when I noticed it rusting I just took it off. Valve works fine without it, it is only there to hold the handle down in place.

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Just a side note. It's not the iron that would harm anything in your tank, at least not at the levels a small screw would give. I would have been more concerned with the other heavy metals, if present, that would also be leached into the tank from that same screw. I agree that taking it out is a smart thing to do.

That was my main concern because who knows what other metals or impurities leach in. Still, if we spend all this money on different types of skimmers, filters, RODI unites, etc. just to get the best water quality, I just don't see why letting something like rust get into the system when its such an easy fix.

I believe one of the issues with iron in a fish tank is that rust is ferric oxide. As in oxygen will leave the water column and bind to the iron. Given enough iron, small enough water volume, or inadequate air exchange and fish hypoxia could be a problem.