From what I've read people let algae grow in the aquarium and largely leave that up to "filtering" the water that and water changes I guess. At least until they are bigger.
I was sceptical at first so I tested it on some daphnia. I took my 20l tank filled it from the hose no dechlor added and that daphnia colony is still going; I only use water from the hose never bother with dechlor.
I dose weekly and my plants don't suffer, I also don't use co2. You don't have particularly powerful lights so I doubt you benefit much from adding co2 I'd just stop doing it all together. Your lights are on too long though.
What do you want this sump for techen? Mainly to hide equipment or to add extra water volume because if its to hide equipment the kent bioreef (which i believe is the tank you are getting) has an area at the back for that.