I would suggest you to empty the reservoir at the bottom, flush with clean water and again empty the reservoir. Now seal the holes of the reservoir with tape so that the water cannot flow out and stays inside the pot. 3% is strong and it may damage the root tips, so i suggest, you dilute 1 part H2O2 with 2 parts water i.e. to 200 ml water add the 100 ml H2O2 and immediately fill the pots with this mixture. You should observe a lot of bubbles and fizzing at this moment. After the fizzing stops about 15 to 20 minutes, remove the tape from the reservoir holes and let it drain out, no need to wash again as the peroxide is converted into water anyways. But remember to wash the next day as the algae will be dead and if not removed will decompose and smell.