If they use the CAM-cycle I guess it would make sense to keep dosing CO2 in
the night period??

No, these are slow growers and have lower CO2 needs as a rule.
There's also no algal or C3 plant uptake during the night so there's
no competition except among themselves which is not that much unless
you have a tank packed with Isoetes and little else etc. As slow
growers, they tend to be mixed with other fast species that dominate.

The only common CAM plant kept is Isoetes which is a heterosporus fern
and it's weird even in a weird group. They also get most of the CO2
from roots(60% up to 100%) substrate.

One plant in the family has no stomata at all and gets 100% of the CO2
from the roots. It's rare and lives up in the Andes of South American.
Stylites andicola.

C4 is more prevalent in aquatics(Hydrillia, Orcutt grass etc).
There is also bicarbonate usage which differes from C4, the effect is
rather similar. It suppresses the oyxgenating activity of Rubisco(the
enzyme that fixes the Carbon) and lowers the CO2 compensation point.
CAM plants do this _temporally_ within each cell. C4 is _spatial_
within the plant rather than temporal.

But adding CO2 during the day is the only thing folks need to do.
Regards,
Tom Barr