Generally yes, with a few caveats. It needs to be completely cured. Depending on the makeup,of the concrete/cement, it may change your water chemistry, similar to limestone type rocks. But is it safe, yes, fish and plants will do just fine
There's some SW guys that make a lot of their rock from concrete. Like stated, it needs to be cured or it will not work. I'd soak it a few days and do before and after water checks to see if it changes anything.
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