My first seahorse tank was a 35 gallon. I haven't had anything smaller but one thing you do need is a protein skimmer rated for at least 2 times the water volume so that it can help clean up the vast amount of detritus they put into the water. My seahorse system is a very simple one with a bare bottom (so I can keep it cleaner by vacuuming often), a regular florescent light bulb, good hitching posts that don't have small spaces for them to get stuck in a few power heads and rock in the sump. A 29 gallon would probably do fine with one nano power head for better flow. You do not need a heater as they are best kept at temps of 68*F-74*F to reduce the growth of harmful pathogens.
For any of the seahorses currently available in Canada (other than dwarf seahorses) you can only have 2 seahorses in the tank you are proposing as they place a huge bioburden on the system.
If you have any other questions I would be happy to answer them to the best of my ability and, if you wish to come and see my seahorses and setups with your wife and daughter you are welcome to come and ask as many questions as you like.
I am usually home on weekends.
