are you talking about dwarf baby tears (Hemianthus callitrichoides) or non dwarf or full size baby tears (micranthemum umbrosom)?
I've been doing a bunch of research on the subject of non dwarf baby tears (i just purchased some) and forming a carpet with them.
From everything I've read, the higher the light, the faster the growth. Both grow like weeds with high light and co2 injection.
However, Dwarf Baby Tears will quickly form a carpet with sufficient light and co2, whereas Baby Tears are more likely to grow vertically.
If anybody has any info on a Full sized baby tears carpet, please let me know!
Hope this info helped.