Feeding once a day enough?

Short question, short reply: yes.

For most fish the answer is yes. In fact not feeding at all now and then is also a good idea.

Fry need to be fed 4-6 Times/day for optimal growth. The Other fish I feed more than once are discus. For a large fish they have a small digestive system and do better with several smaller feedings rather than one larger feeding. I am sure there are a few other fish that need a different feeding schedule, i just haven't experienced them yet.

I feed mine once a day (unless they're begging and I happen to be in a generous mood) and fast them one day a week. It seems to work out well for me.

Yes, I feed all my fish including SW only once a day.
Rohn