Change the filter cotton wool regularly. Use a good filtration system. Ensure adequate oxygen is supplied to the aquarium. Use appropriate aquarium decoration, allowing sufficient hiding places and providing for any special needs of individual fish species. Do not mix ferocious fish species such as tiger barbs with guppies, swordtails, and lilies as aggressive fishes can attack other fishes.
Some fish like to go in groups or pairs, others in singles.
Lilies seem to prefer singles. If two lilies are to be grouped together, ensure that adequate shelter is provided for each lily. For example, give a bridge to one lily and perhaps a small castle to the second lily. Do not overfeed the fishes. Feeding them two times a day is sufficient.