Afternoon,
Well being a developer/supplier of a PDA Tank Manager, I can offer a little insight to this topic.
You have to decide if you want a desktop (windows) or a PDA version first. Currently I know of no software out there that is combining both, though I am releasing a desktop editor for my software in the coming weeks/month or so.
If you are looking at Desktop software, there are two that I would recommend you look at. The first is ReefCon 2000. It is a very comprehensive software package and offers a lot of features. They even have feeds for some tools like
wave makers and other pieces of the aquariums setup and hardware.
ReefCon
The other is Aquarix.
Aquarix
Aquarix is a nice one since they offer a huge database of photos to attach to your system, along with feeding and other information.
On the PDA side of things, I am obviously biased to my own product. There are only two software offerings out there on the market that I am aware of. Mine and my competitors.
AquaTrac is my product and I feel it is the most comphrensive and sophisticated offering. It offers unlimited numbers of tanks and is completely customizable for readings and task management. I offer feeds into the DateBook and ToDo programs that come with every PDA (PalmOS) out on the market today. It is geared towards the reef or saltwater crowd, but I have been adding more and more functionality geared towards the freshwater and pond enthusiasts as well. I also offer the best customer service around

The only feature I have yet to add is onboard graphing and that will be added in the near future. It is also in full color and offers many utilities for exporting to excel and csv formats for use in any number of windows programs. It also includes printable reports for your computer to keep hard copy records. I also update and improve it on a regular basis.
AquaTrac
The other PDA offering is called AquaPilot. This was the original software out in the PDA market. The interface is a bit confusing and awkward to deal with, doesn't offer color and from the reports of some of my customers, the support is lacking. It is a bit cheaper than mine and does offer graphing, however it is not very feature rich. There is no interface with any windows software at all.
AquaPilot