Would it be possible to shim this or should I get a yoga mat? It’s a 40 gallon breeder. The gap is maybe 1/8th inch.
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I would use a self levelling mat for the tank
https://www.bulkreefsupply.com/auqa-gadget-leveling-adhesive-rubber-mat-innovative-marine.html
The tank/stand are touching on 3 corners but on 1 is not, so putting a piece of plywood would still leave a gap.
I used plywood and a matShim between the “low corner” frame and the 3/4” plywood to close any potential gap between the tank and the plywood. For rimmed tanks, those self leveling mats are useless as those mats are meant to be used for rimless tanks. Using the 3/4” plywood top is your best option IMO.

