I'd like to make an anecdotal report on my recent experience with different substrates.
My 5 gallon I used straight topsoil with fine gravel covering. I experienced lots of pearling on plants initially followed by very slow growth and both Echinodorus tenellus and Sagittaria sp. putting out transparent, colorless leaves that eventually, after weeks, turned green. I also had a fair bit of hair algae which eventually calmed down. Similarly, I have a goldfish bowl with Heteranthera zosterifolia that took many weeks to do anything. It's now healthy and slow-growing under fairly low light with cold water.
For my 20 gallon, I mixed equal parts of the same, local clay-loam topsoil, Fluorite and washed Leonardite. I also added Azomite. Same, fine gravel on top. This was in a possibly foolish attempt to give my substrate everything it might need and not rely on whatever my topsoil has or hasn't. I am getting much better plant growth so far. I have not experienced any yellowing of the water from the Leonardite.
How much this has to do with the volume of water, I don't know. The other differences are I am not heating my 20 gallon and it is illuminated by LED, rather than CFL, same color temp tho'.
For those interested, left to right I have Eleocharis acicularis, Hydrocotyle verticillata, Sagittaria platyphylla, Lobelia cardinalis, Bacopa monnieri, B. caroliniana, Samolus ebracteatus?, Ludwigia palustris x repens?, Echinodorus tenellus, E. cordifolius "Harbich", Polygonum hydropiperoides (not really visible in photo), Potamogeton illinoensis and Myriophyllum heterophyllum.
I highly recommend collecting plants where possible and legal, it is so much cheaper and more satisfying.
I have added many pond snails, that do a very good job of cleaning off the calcified coating on all the plants in my swimming hole. I also have a couple of red spider mites that swim around. I plan to stock with Florida Flagfish, perhaps 1 male and 3 or 4 females.