Bryon is right as he did a lot of studies/research plus his many years of experience in fish keeping.
I won't be going into the details as there are too many things to explain.
But just to explain very briefly.
In an aquarium(confined environment), "soil and fish" are not recommended to be together.
In my opinion, soil and fish are not meant to be together in an aquarium though I know that there are many people who are doing it especially those who are doing aquascaping.
To me, their main priority is the well-being/health of their plants than their fish.
But to me, my fish health/well-being are the most important and not the plants.
So, my advice is to keep "fish with plants but without the soil" or "plants with soil only".
Some soil will create ammonia spike for the first few weeks.
Some soil contain heavy metals, chemical, etc that are bad for the fish and may kill the shrimps or any sensitive sea animals.
Soil will also change your water pH.
And the worst part is, soil will gather/accumulate all the debris, fish waste, left over food that are hard to be removed from your tank. It's very messy and hard to maintain if you have soil in your tank.
Lastly, please bear in mind that an aquarium is a confined environment unlike in the wild/rivers where the water is being renewed frequently.
Any waste, toxic in the rivers will be flushed away frequently.
Anything that goes into your tank will accumulate to a dangerous/toxic point that will kill your fish unless you change the water frequently.
Here are some interesting articles:
Looking to build your own fish tank hydroponic garden? Learn more about aquaponic gardening, an efficient combination of a hydroponic garden with fish tank.
www.motherearthnews.com
Fish belong in their natural habitats, not in your home. Here’s why it’s cruel to keep fish in tanks and what you can do if you already have them.
www.peta.org