Welcome to Tropical Fish Keeping forum.
I second what has been suggested by others, and will add a couple cautions. Taking the sunlight first: Never allow direct sun to strike an aquarium. First, there is the temperature issue; sun heats up water fast, and this can cause real distress for some fish. Second, the brightness of sunlight is something that is foreign to most of the forest fish we keep, that come from quite dimly-lit waters. Fish are not designed for bright light, and this can cause severe stress. Third, from the aquarist's perspective, a sun-lit aquarium is not really easy to view or see clearly.
A substrate is important for fish, as was mentioned. The darker the substrate, the better. Aquarium glass reflects light internally, and this can be very unsettling. There is also the very important function of a substrate biologically. There is good reason for bare tanks when it comes to certain situations like treating a sick fish, but this should never be the norm.
Stress is the major cause of all fish sickness, and stress will occur from any of the above as well as many other factors. Preventing stress is the only way to have healthy fish.