Do you need fish at all?
Short answer, no, not really.
Many people assume corals are the hard stuff. Some are. Many are so easy you have to be careful or they'll be like crabgrass.

One of the problems people have as NTTH folk (new) is assuming fish are tough. Most marine fish aren't that tough. Not all corals are. But some corals are kill-them-with-flamethrowers tough.

Corals do emit waste, but moderately so. They'll live on light (literally: their colors...