T5 refers to the width of the tube - it is 5/8 inch wide. The other common size is T8 which is 8/8 inch wide, ie 1 inch exactly. They are not interchangeable. If the connectors on your tank are for T5s you have to replace them with T5s.
Marine bulbs are usually a different colour from the bulbs needed by freshwater fish, often bluish. Maybe someone else can say for certain, but I don't think marine tubes are a good idea on freshwater tanks.
RichardW also recommends using general household fluorescent tubes as they are much cheaper than those made as aquarium tubes.