People have used everything from yard soil to pond soil, and get all kinds of little critters and other organisms with them.
Some seem to be harmless, others aren't.
You'll see pictures of some on the threads...
Don't use the fake stuff - get the real dirt.
Apart from anything else, the natural soil bacteria will help with the tank processes, while nasties seem to expire or be starved out in the packing process.
Plants grow in dirt - and it's worked forever - as far as life itself goes, anyway.
Best bet, cheap topsoil - no added anything, and all unbreakable lumps of organic matter carefully removed.
No chemicals, no vermiculite or perlite - and check out the planted section for a comment entitled: 'The Cost of Vermiculite - Breathtaking' if you don't know why.
Also, stuff like that floats up into the water...
Do NOT use any Scotts product - they're loaded with toxic chemicals and heavy metals - someone on one of the El Natural threads had some info on that, as you may or may have have seen; I don't know how global they are, but they're pretty big, and may, for all I know, ship poison to Belgium as well.
Some peat content's OK, but apparently too rich is liable to translate into an ammonia/algae farm, and wetting agents used on this for potting soil can be, I believe, (too lazy to check right now) deadly to fish.
Not good, anyway.
If you could only get topsoil that could be used in organic horticulture, that would be ideal.
Even if much more expensive, for a small bag, and for the increased health of your tank, plants, fish and all, it'd be worth it.
I was lucky enough to be given by my Dad a nearly full bag of lovely soil which did have some peat in it but which had been open for several years, and left out in the elements for a couple of them.
This was perfect, as not only was ammonia released, but a fair number of chemicals would have been broken down/removed, I'm told.
So if you can get an old, opened bag from anybody, ever so much the better.
I was able to contact the business, a smaller, local, privately owned outfit, explained the use I intended the soil for and my concerns, to be told that their soil came from farmers fields, (so would have been artificial ferts added at some point) but that they hadn't added anything.
And I went through everything I took from the bag, removing anything hard/lumpy, along with the moss growing in it, for every tank/potted plant done.
And it's important to remember to at the least let that bag of soil air out open for a bit, as quite a bit of ammonia/nitrates will likely gas out, and it helps if you can spread the amount you want out flat for a few days.
I swiped an enormous, relatively shallow metal mixing bowl that was stored unused in the laundry room - if you can find something like that, you'd probably find it as handy as do I.
The soil could be spread out in that fairly thinly, without risking spillage, and left to air out for a few days.
Or there's a thread on 'mineralizing' the soil which involves soaking and working it, if you're interested.
But it's the people who have the wrong soil, and/or don't have aired soil, who develop ammonia spikes and algae breakouts in the early days.
And it can take months to stablize in that case - when care in soil selection, and even a few days airing, can make so much difference in producing a trouble-free tank.
edit - just thought I'd better add, in case you don't know, that I believe you need about 40% plant matter, to assure good uptake of ammonia as produced, and do need to aim for about that amount or more to grow in, and will of course, need cover plants, something low, for the front so as to have a swimming and feeding area where your fish will be visible.
That's fairly easy to do in the small tank planned.
