Any reservoir that hooks directly to the Eheim 1046 fits on the AS XT too. Aqua Computer has the aquainlet, Innovatek has the AGB-O-Matik, and there are several others. About desirable: that's a question of personal style. I had the AGB-O-Matik and I didn't like it. But that's just my opinion. One downside is that to remove the pump or the res, you'll have to remove both. One upside is that it separates easily from each other at the twist of a hand. Make up your own mind. Piece of advice though: the AGB-O-Matik is very difficult to close tightly, also it's hard to get the air out of it and the cap doesn't live long. I'd recommend getting one of the others if decide to you go with that principle.