CO2 on a crypt only tank? Bit of waste IMO
Nutrious substrate & don't blast it with excessive lighting
My C.Balansae is 50cm+ after a couple of months, in a low tec.
Mid ground C.Tropica & C.Undulata for contasting leaf shapes.
Front C.Parva, or Amoricum, or Willisii
A lot of Crypts seem to do...