The Choice: Freshwater or Saltwater
There are millions of different environments out there in the wild and with it millions of different species of fish to go with it. This kind of variety is very hard to represent in such a small space as a home based aquarium. It is for this reason that having a culturally diverse fish tank is next to impossible. The variance is simply too high. This is in part because one living condition may be vital to one fish may have a negative impact on the health of another. In short, putting the wrong fish in the wrong living condition may well kill the fish.
This is the reason that makes the decision between choosing a fresh water and a salt water aquarium is so important. Choosing the wrong one for the type of pets you’d like to keep would be a bad omen. In addition, mixing and matching is a bad idea as well as only a fraction of your fish may survive and the rest may not do so well under the sickly conditions.
Freshwater fish are adapted to living in low salt concentrations, (i.e. less then 1 %). They have a very difficult, if not impossible, time adjusting and living in salty conditions and should therefore not be mixed. These fish generally come from lakes, rivers, or streams instead of the ocean which most people think of as aquarium fish.
Saltwater fish on the other hand have a high adaption to salt and those they want to life in a saltwater biome. It is important to note that while a saltwater fish can live in freshwater, this living condition not optimal for health as is considered a risk to the fish. The origins of fish of this nature are from the ocean and a large proportion of aquarium fish of this type are typically tropical.