I think an aquarium would be soothing for you and interesting to your cats. I like Lut's suggestion that it have a very secure top, too.

I'm not fully awake yet. When I first read your post I was thinking of one of those large public aquariums, and very confused picturing you standing in line with your cats to buy your tickets. Wondering what sort of pricing issues would come up for the cats and doubting they'd even be allowed in. Finally caught on when I got to Randi's post. [But it was an interesting mental picture!]