I've added drained eco complete to an established tank several times without any real problems. I drained it 'cause the liquid was just too much to hassle with when I was originally storing it, but I don't think it would matter for adding it to the tank. To avoid getting the small bits of EC dusting everything in the tank I used a container to scoop some EC up, lowered it into the water upright, then tilted the container and emptied the EC where I wanted it - SLOWLY. Go too fast and you get a cloud of EC that spreads across the tank (voice of experience speaking here). I used a large cottage cheese container that I'd cleaned and rinsed well, you might want to use something larger since you'll be adding more. A little patience and care will help keep the mess and the amount of dusting off of the plants to a minimum.