I have tenellus in my quarantine tank and i've had subulata for a long time in my Rio..
Leaves look a little different , tenellus starts slimmy , than wide and then pointy... ...while sagitaria goes from wide to pointy.. (hmm, i hope you will understand my poor description)
+2 on the sagitaria. That stuff took over my whole tank (65g) about 5 years ago. It sends out runners in every direction like vals do and if you dont constantly prune them youll have more than you can deal with.
The picture I posted was taken by the seller who called it pygmy chain sword. It just arrived today and it looks like Echinodorus tenellus but I'm not sure. The seller's picture of the plant didn't seem to represent the plant as I see it when it arrived. Comments?