24", is this a 15 gallon? either way it sounds like solid medium light, which opens you up to a lot more of my plants. You should be able to grow dwarf sag, rotala rotundifolia, hygrophila polysperma, l. repens, giant bacopa...probably a couple more, but it's late and my brain is tired. lol
No a 20g long tank, its about 30" long I think. Anyhow riccia is priority one for me, if there aren't enough clippings of the rest that is fine.