I used tires in my pond 15 years ago and so far everything is OK
I tied my in a pyramid shape similar to the shape below using polypropylene rope. It's a synthetic and it doesn�t absorb water, it resists rot, mildew, oils, acids, etc.
I put my pyramid together on the ice and it dropped through when it melted. I drilled holes in the tops of all the tires except the top two so air wasn't trapped in them. I filled the bottom tires with rocks to sink the tires and left the top two tires with no drilled holes so the air collected in the tops held the tires upright when they sank.
It looks like my little drawing isn't going to show up in a pyramid when I submit this but you get the idea.
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