anyone make a path with different colored substrate before? how do you stop the two from mixing near their edge
Use small parts of plastic or glass siliconed at the bottom of the tank to prevent them from mixing. Note that this won’t stop fish who move substrate via mouth like cichlids from redecorating what you had made.
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Exactly, using either silicone pieces to separate it all out or possibly making sure that you have fish that will not mix it around since many will as a territory aspect or may when they get scared.
You can also use gravel instead for both which will prevent them from mixing as easily as sand will throughout the time.