RE: 5g killi tank substrate
Peat and coco coir can be used as a primary substrate for grow out tanks. Again, it comes to preference and how your water is. I've seen people use tiny amounts and people that have had 100% peat as their substrate. Ive personally never tried soil substrates so that isn't a question I can confidently answer. I can tell you, however, that soil alone wont insure healthy plant growth, but a combination of all the components of your tank.