Check your water parameter, it should have 0 ammonia, 0 nitrite. Some nitrate presence is not lethal as long as its within the limit.
Match the pH and temperature depending on the species you plan to put it.
Water should not be cloudy
There should not be any bio-film forming on top of the aquarium. In this case increase surface agitation.
Make sure your tank has enough filtration to support the bio-load.
Tank decoration should match the requirement of the species. Some prefer less open area and more hiding place, some prefers the opposite. Some prefers plants, some prefers rock, some prefers a piece of wood.