Yes, you can make seawater out of fresh water, but it is in no respects cheap and easy, because you need a lot of technical equipment (e.g. protein skimmer (if that's the correct name in english), filter, heater and/or cooler and very large pumps to make something like waves or current). And for orcas you had to do all this on an enormous scale - they are no clown fish 
- and they need a bigger tank than your "normal" 2000 l reeftank.
And even today most saltwater fish kept in tanks are taken from the wild and not from nurseries.
- and they need a bigger tank than your "normal" 2000 l reeftank.And even today most saltwater fish kept in tanks are taken from the wild and not from nurseries.