Sample the Water Your Corys Breathe
Most keepers dip a tube at the surface, but a corydoras spends its life in the bottom layer, where uneaten food and waste settle and decay first. Draw your test sample low in the tank, just above the substrate, so the readings reflect what those fish are actually breathing and feeding in. Use a liquid drop kit, rinse the tubes in the tank water, and test just before the weekly water change for the least flattering and most useful picture. Read each colour at the time the kit specifies, in daylight or neutral light, since the bottom-layer story is often told by a borderline ammonia or nitrate result you need to read honestly.
Ammonia, Nitrite and Nitrate at the Bottom
Ammonia and nitrite should both read zero. Any ammonia in a cory tank usually means the substrate is holding more waste than the filter and changes are removing, and corys show it early through worn barbels, redness around the mouth, and fish that stop foraging. Nitrate should stay low; lift it down with partial changes and by vacuuming a section of substrate at each change rather than stirring the whole bed at once. If readings are poor, siphon gently over the sand and watch how much debris comes up. That debris load, more than any single number, explains why bottom fish suffer in a tank that looks clear from above.
pH, Temperature and the Substrate Itself
Corydoras suit slightly acidic to neutral water and a steady tropical temperature, and tank-bred fish handle a modest range well as long as it does not swing between tests. Confirm temperature with a thermometer at the lower part of the glass, not only near the heater at the top. Testing day is also the moment to look at the floor itself: fine, smooth sand lets corys sift and forage without grinding their barbels down, while coarse or sharp gravel slowly injures them in ways water tests cannot reveal. If barbels are shortening while readings are clean, suspect the substrate and the debris within it before suspecting disease.
Oxygen and Behaviour Complete the Check
Corys make occasional dashes to the surface for air, but frequent, frantic trips by the whole group suggest low oxygen or irritated gills, both of which pair with poor bottom water. Check that the filter return ripples the surface and that no fish is gasping. Feed a sinking food and confirm it reaches the bottom past mid-water tankmates, because a cory losing condition in tested-clean water is sometimes simply being outcompeted. Act on bad readings with partial changes of conditioned, matched water and a substrate clean, then retest the next day and watch whether foraging and normal shoaling resume.
Keep a Bottom-Level Log
Record the readings, the debris you removed, barbel condition, and how actively the group fed, all with dates. Cory problems develop slowly and are easy to rationalise week by week; a log shows the nitrate creep or the gradual barbel wear that memory smooths over. If a cory still isolates itself or its barbels inflame in water that tests well, that written history is what an aquatic vet needs to look beyond the obvious. Holding the log in KinsLe in a Pet Passport keeps the corydoras group's water record and health notes together for that conversation.