It depends on the tank size, depth, and if you have other things that will "match" up to the light. A general rule for a planted tank is about 3w/ gallon, but higher lumens means deeper penetration unlike fluorescents. One other difference is if you have actinic bulbs. Fresh water tanks tend to use 6700k to 10k full spectrum bulbs. Some are also red/pink which green plants absorb better. If you have 3 or more watts per gallon you need to be able to match it with a good soil substrate rich in iron, pressurized CO2 (which you might already have since you had SW), and fertilizers that will provide both micro and macro nutrients.
If any of these is lacking you could run it to various algae problems. Another general rule is to overstock a new tank with plants. With this in mind you can also get low, medium, or high light loving plants depending on your watts/gallon from above. If you don't want to be bothered with all of the above, you shoot for a low light tank (less than 3w/gallon). Hope this helps.
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