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Looks a little more like stargrass to me.
Guppy grass (Najas guadalupensis) has very brittle yellow to brown branching stems and will grow floating and it will send out very long tendril like roots that easily work their way into sponge filters, under rocks etc.
Stargrass (Heteranthera zosterifolia)can grow floating but more typically grows as a leafy stem plant w/ green, very flexible straight stems (not much branching except where pinched off).
Basically, if you can pull an entire stem out of the water w/o it breaking at the least little hangup, it's stargrass. If it breaks easily, it's guppy grass