What impact on biology!? Their lifespan you mean!? The 20 gallon tank can limit their lifespan!? And till how long can they be in the 20 gallon?!I'm not sure why the answer to this question keeps getting missed because three or four of us have stated it, but that's OK. The 20g is "OK" for the fish now in it while they are still small, but it is not suitable for the present fish as they grow. And hopefully they will, if healthy. Mollies at 3-4 inches in this tank is a lot of impact on the biology, even ignoring the physical space.
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