I use one of these, the Turmix AX 200. It’s exactly what it looks like, a tub of water with a heating element. I get limescale building up, but no algae. Personally, I wouldn’t use an ultrasonic one, because I don’t care to have ultrasonics blasting in my ears, even if I supposedly can’t “hear” them. Loud ultrasonics can damage your hearing even if you can’t hear the ultrasonics themselves, and I’ve no way of measuring how loud they are.
I use one of these, the Turmix AX 200. It’s exactly what it looks like, a tub of water with a heating element. I get limescale building up, but no algae. Personally, I wouldn’t use an ultrasonic one, because I don’t care to have ultrasonics blasting in my ears, even if I supposedly can’t “hear” them. Loud ultrasonics can damage your hearing even if you can’t hear the ultrasonics themselves, and I’ve no way of measuring how loud they are.