If I built a concrete pool, would it ever contain fish?
if the pool were just sitting out in open air, on earth and all that good stuff, it certainly would, given enough time. 

This thought experiment is in the same vein that inspired Darwin when he reflected on his experiences on his visit to the Galapagos islands.  

Think about it - the many islands which are formed by upheaval were lifeless when they first emerged from the ocean.  Over time, the winds or storms eventually populated them all - just about every island has animal life on it.  Not all animal types are found there that are best for the climate; its a matter of accident that some birds or lizards are found on the galapagos islands.  These accidental landings don't happen very often and sometimes a single animal will generate many species which will fill the role that other animals would normally fill on the mainland continents at the same latitude.

This is what happened on the islands- in different islands, Darwin found finches of sizes and shapes would be eating insects from bark or ant hills that might be done by an animal 

Eventually, if you wait long enough a frog, then maybe a couple of fish will find their way into the pool from a nearby waterway (yeah that could take a long time I know).  If it takes too long then a new species will show up.  If you build walls around the pool it might not happen.  These accidents of animals showing up by accident can take millions of years. 

This experience of Darwins' has inspired an entire field of biology - Island biogeography.  Each island is an experiment and ecological niches can be observed by taking a series of islands and seeing how different animals take up the various ecological roles on each.  A famous experiment EO Wilson and colleagues did was to fumigate some mangrove islands and watching them repopulate.