Hello Jacob,
I hope I understand you right: the plants arrive in plastic pots with planting medium and that you want to cultivate the plants in pots (plastic or clay pots?). Plants are often put in rock wool. The rock wool contents a lot of P and N and it looks very ugly. So you need another substrate. And please use clay pots.
I use special subtrate for plants (JBL Aquabasis) in two Aquariums to cultivate Echinodorus. This product is sandy clay with a little bit of peat. Above the substrate is a layer with gravel. The substrate does not effect hardness, pH or N and P content in the water and the plants grow well. I think the substances sand and clay 10:1 and a little bit of peat or leaves and perhaps some ashes of wood should be good for cryptocoryne too.
Blackworms are very good, but perhaps you can cultivate and feed Daphnia or Moina too.
Greetings
Patrick