Most [plants stop photosynthesizing after 12 hours, getting ready for their night time period when they actually use O2 and produce CO2. You'll notice most stem plants at the end of the day will have their leaves fold upwards closer to the stem. If they don't get a rest period, they stop their growing period. If they're not growing, the algae is. I won't even consider a planted tank without a timer. You want the regular light period.