as there is no light, and with no light and a lot of Co2, your plants will not grow that well, and algae will most likely show up.
That doesn't make sense at all.
Plants will indeed respire at night, where they take in O2 and lose CO2 however that doesn't mean that the plants won't grow well. Plants need a night time so they can respire as this is important to the plants health. Also, you wouldn't get algae because algae needs two things, ammonia and
light. The plants aren't going to be breaking down and leaching ammonia at night.
High amounts of Co2=Bad for fish.
Myself and others haven't had problems running CO2 24/7. It is recommended to have a slight ripple on the surface for good gas exchange, this usually prevents build up of CO2 over night and incorportes O2 into the tank (plants have been doing that all day too). There are folks tat turn CO2 off at night but turn it on 2hours before the lights come on. This is mainly to save CO2 though rather than save gasing the fish. Others run airstones or raise their spraybars/lily pipes at night just incase.
Remember, high CO2 doesn't nessaarily mean low O2