What goes together and what doesn't?
About 90 % of all aquariums are community aquariums. Fish, plants and often invertebrates are kept together in one aquarium, which come from different regions of the world, but whose requirements fit together well. The contrast to this would be a biotope aquarium, in which (with strict rules) all animals and plants come from one area.
However, there is absolutely no reason why you shouldn’t put animals and plants together if they get along and have similar demands on the water and the equipment. It really is best to find out all about the animals and plants BEFORE you buy them or to listen carefully to the salesperson in the pet shop and take their advice. There are fish that require extremely different water values and unfortunately this is not always taken into account in the pet shop. For example: cichlids from Lake Tanganyika prefer quite hard water, while the cardinal tetra is a soft-water inhabitant.
Fish species can often live perfectly well in medium-hard water at a pH value around 7.0. Avoid any extremes in one direction or the other. Fish which regard other fish as prey also do not go well together - at least not for long! Fish that need a lot of swimming space, e.g. rainbow fish, do not go well with fish that prefer a more weedy aquarium (e.g. gouramis), even if they both have the same water value requirements.
The water temperature is another factor to be taken into account in the community aquarium. Many shrimp species prefer colder water (21-23°C). The discus loves warm water (27-29°C).
Besides, once caught, the discus would eat the shrimps. With a water temperature around 25°C, most tropical fish species can really be kept well. Only a few species really need a higher or lower water temperature.
Another thing that sounds funny but isn't: When the film "Finding Nemo" hit the cinemas, crowds of people flooded the pet shops to buy nemos (clownfish) for their aquarium at home, not knowing that clownfish are saltwater fish but their aquarium at home contained freshwater.
We actually distinguish clearly between freshwater and seawater (with about 40 g sea salt per litre). In between is brackish water. In fact, we cannot easily identify a water sample as seawater at all unless we taste it! Only the general hardness and the conductivity (measurable with a conductivity meter) would indicate seawater. All other water values would also be suitable for freshwater!