Sorry, I think the answer is in here, but I'm not sure I've found it so I want to ask. These seem to have the bubbles coming out of the bottom, but need a few inches of water above them to make them work properly. So if I want one in my sump instead of my DT, I need to make sure it is short enough, or create a baffled area with some extra height. "A" is the right size for my DT, but I need to make sure I've got ~ 9 inches of depth - correct?
Also, I could see getting one for my QT. That would be a D, but it would go right in the tank, so an A would fit fine. Other than cost, and I suppose risk of knocking it over and overdosing, would that be a problem? (eh, suddenly the D seems like a much better fit based on those 2 things alone).