For total number of fish, the rule of thumb is 1" of fish per gallon. In other words, in a 20 gal
tank, you could have 20 one inch fish, or 10 two inch fish and so on. If you are talking about a single fish in a 29 gal
tank, obviously the rule will not apply. A 29" fish would not even fit in the tank. Basically, you want to make sure that whatever the fish, when full grown, will have plenty of room to swim and turn.
Just curious, why are you asking?
EDIT: My mistake. I have just ventured over from freshwater. That should be 1 fish per 5 gallons.