Getting into FW

[Fugu]

If you really want to go big you could go with co2.But it is for super serious planted tanks.
You could use your SW filter,but what is in it?You may need to replace the sponges and the other media.
I SERIOUSLY DO NOT RECCOMEND SNAILS,well trapdoor snails.They breed too much.Unless you have a SW puffer tank.If then go for it
All I use for filtration in my planted tank is a co2 system.But I keep fish.If you want to go all plant with 3 or more medium fish then you don't need filtration.Plants,catfish,shrimp and snails are the best filters for a FW tank.