Ah its probably just doing a hammer thing. I wouldn't worry. It still looks happy. Sometimes they expand, or contract, or poop, or whatever.
Most people use clear mesh with a window screen kit. I use Lexan or bullet proof glass lol Unless your fish weighs a good amount they wont be able to jump and knock the lid off. I ran my LEDs through the lexan. no open top. gotta keep cats and dog hair out.
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