perhaps it would be helpful to point out the difference between two kinds of "topsoil". Some use dirt. Like, go in your back yard, pull out some carrots and onions for dinner, then scoop up some of the garden into a bucket, and toss it in the tank for your sub. (search youtube for "dustin dirt tank" you'll find a very avid fan of that method)
Some use "top soil" meaning stuff in bags from homedepot, lowes, etc. which can contain a myriad of things. Some of which are more desirable than others. Using a top soil that has lots of chicken manure mixed in, may not be so good for your tank right at setup (ammonia much?)
Regardless of which you mean, people use both, with and without caps. The caps are mostly for cleanliness.
If you buy from the store READ THE INGREDIENTS and do your research on them. Some contain peat, not a big deal if you dont mind the ph change, and tannins in the water. Some contain animal manure, which will at the very least contribute quite a bit of ammonia. And others contain virmiculite, those tiny white rock things. At the very least they are annoying when they collect at the top of your tank (they float). Also be aware of inorganic fertilizers which may not be good for your choice of fauna.