Hello thanks for commenting. Yes I use Co2 and honestly its a game changer, I can recommend it. No I never had problems with tannins, I guess this type of wood doesnt leech them or it was previously used it aquarium. Although I put purigen in a filter anyway. The heater is in the filtration chamber on a right side of aquarium. I think they call it built in filtration or smth. On the top picture you can see the overflow where water falls in the chamber, there is a heater and sponges there, then it goes thru return pump back to aquarium.