i have this guy on here saying i can use this stuff for my planted tank if i put gravel over it it will keep it down and keep from clouding i don't know if i fully believe in this anyone else try
yo i have soil in one of my planted aqariums. I would try this on a small tank with nno fish in it. Take the top soil put it in, in about an inch worth of water than place the rocks on top(gravel)then fill the tank up as usual but if there is clouding turn off the filter and wait about a day any more questions call pm me
I've had my tank set up this way ever since I started keeping fish. Also, I know a guy that ONLY has this set up, and the only reason he even has fish, is to fertilize the aquatic plants.
I have two tanks set up like this. They look great not a lot of work and my fish love them.
I got my info here http://www.aquaticplantcentral.com/forumapc/el-natural/ and I have the book Walstad wrote about it.
It can work. My first attemp was a bowl and it turned out great. So I went bigger and bigger. My biggest one is 29 gallons.
Here is a great way to start out and see if you can make it work for you. http://www.atlasbooks.com/marktplc/00388Shrimp.pdf
I would not do it on a tank that size. I did it in a 10 gallon,and it took weeks and a lot of water changes for the water to clear up. It's not that you couldn't do it in a 125,but it will look pretty bad for quite awhile.
You could add your fish right away if you do decide to do it. Just make sure to put plenty of gravel on top or you will end up with a muddy mess.
I re-planted my 55-tall and put the fish back in the same day. Just keep an eye on the water parameters. I haven't had any problems with mine and I just use regular generic top soil.
A woman here in Oklahoma has a 125 planted tank with multiple featherfins, corydoras, angelfish, plecos. It's all natural, no outside filter, just some powerheads for water circulation.
It will take a LOT of maintenance out of owning a 125.