Smell, or the chemical detection equivalent on "feelers". Just watch your fish cruising around the bottom, and do tighter circles till they zero in on a shrimp pellet. Watch the cories and plecos reverse direction when they have swam by food. Definitely more than sight.
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Oceanic 55+,two Fluval 404's,In-line 300W heater,Moderately planted with Polyvinylus Chloridus sp.=the only plants that last in my tank