the problem isn't just the size, it's aeration and filtration. both of these are usually accomplished by hang on the back filters. in a bowl you can't really have a good filter.
is you bowl heated? if not, get a 50watt heater for it.
as for the filter, a good sponger filter might do the trick as long as you don't add anymore fish and you keep up on your water change (25% once a week).
