Ask whether the food will reach the bottom
Ask a sitter whether the food they give will actually reach your corydoras. Corydoras feed on the floor of the tank, sifting and picking with sensitive barbels around the mouth, while quicker fish above them can finish floating or slow-sinking food before a single piece lands. Ask if they will drop in the sinking wafers or pellets you supply after other fish have been fed, and watch for a few minutes to see the corydoras gather over them. A sitter who assumes the catfish will clean up leftovers is a risk, because a group living on scraps slowly loses condition while the tank looks well fed.
Ask how they will watch the group, not just the tank
Ask how the sitter will watch your corydoras as a group. Corydoras are social and normally rest, forage, and dart to the surface in loose company, so a single fish sitting apart, breathing fast, or no longer joining the evening shuffle is the early sign worth a phone call. Agree a simple daily count: how many corydoras should be visible, and what to do if one cannot be found. Ask them not to go digging through ornaments or stirring the substrate to hunt for a missing fish, because that panics the group and clouds the water. A calm photo sent to you will usually settle the question faster than a search.
Ask about hands-off substrate and filter care
Ask whether the sitter is willing to leave substrate and filter care exactly as you set it unless you have demonstrated otherwise. Corydoras barbels are easily damaged by rough handling and by sharp or dirty gravel, and well-meaning deep cleaning during a short absence causes more harm than it prevents. The useful sitter tasks are surface ones: removing obvious uneaten food after a feed, topping up water with the treated water you leave, and checking that the filter still runs quietly. If a water change genuinely falls due during a longer trip, walk the sitter through your exact method once in person before you go, buckets, siphon, and all.
Ask about sudden surface trips and stillness
Ask the sitter what they will make of corydoras behaviour that looks alarming but is normal. Corydoras regularly shoot to the surface for a gulp of air, rest motionless in a row, and become very active after lights-out, and a sitter who does not know this may either panic or start adding food and treatments the tank does not need. Explain your group's normal rhythm, then name the changes that do deserve a call: repeated frantic surfacing by the whole group, redness or erosion around the barbels, or fish lying on their sides. Clear questions beforehand turn a worried sitter into a reliable observer.
Ask about the other fish sharing the tank
Ask about the tankmates, because corydoras rarely live alone in a community setup. The sitter needs to know which fish get which food, whether any tankmate bothers the corydoras at feeding time, and that no new fish, plants, or ornaments should be added as a surprise while you are away. Also agree what happens in a power cut or if the filter stops, in plain steps the sitter can follow without improvising. Setting that handover out in KinsLe through Care Pass gives the sitter the corydoras group's routine and tank details in one clear place for the whole trip.