You can have too many plants in a tank.
If there are lots of plants and no surface turbulence, the oxygen level can drop at night as the carbon dioxide (CO2) level goes up, the pH can drop, and fish can suffocate.
During the day the CO2 level drops and the pH can go up and the fluctuating pH can harm the fish.
This can be prevented by having aeration/ surface turbulence to stop excess CO2 building up.
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The other issue that has already been mentioned is swimming space. If the fish get tangled up in plants, there is not enough room for them.