I was apprehensive at first and it took me a while to adjust my head into not carrying out water changes too, but now I'm used to it. You'll just have to sit on your hands at the weekend
Carbon products are reputed to kill some types of algae by direct application of it using a syringe, but personally I wouldn't use it in a lo tech because the plants will then have to adapt to the higher carbon content while you're dosing then re-adapt if you was to stop dosing. I've learnt that plants thrive in stable conditions.
Clive advocates keeping it simple. So I did, my whole regime for my lo techs are as follows:
Feed the plants on a Saturday.
Clean filter once a month
How simple is that?
If changing water introduces CO2 which we want the plants to adapt away from in a low light/tech, no CO2 set up, how do you deal with evaporation and dropping water levels. In my little tank (35l) I've a luminaire and a combination of the heat from the light and an open top to the tank means that my water levels drop reasonably fast.
How do I reduce the impact of topping up on the CO2 levels?
well the amount of water you add back into the aquarium will be little to change the CO2 levels, i wouldnt imagine it to be no more than 5% of the total aquarium volume so the CO2 levels dont change drastically, but you can do as chris says and let the water stand or aerate it if you feel more comfortable.