| A) How long has the tank been set up?
B) Are the tablets like wafers? If so don't feed the whole thing to them. With 3 cory's, break the tablet in half and then break 1 half in to several smaller pieces. Trust me that they will find it easier to eat and I will almost guarantee you they won't leave anything behind. When I started algae wafers... my corys didn't want a part of it because they were so big.. it was too much work to bother with. They'd go back to the fake plants and eat algae off that instead.
C) Back to A, if your tank isn't already cycled then corys are quite sensitive to water conditions and could be damaged long term because of that.
I would not expect tablets to cause huge day to day ammonia swings, but in a cycling tank I wouldn't doubt it. If you weren't doing weekly water changes then I could believe that the ammonia levels were swinging on a weekly basis, but even then it'd be hard to say. Definitely change the amount of food you are giving them as it certainly can lead to an ammonia build up, and int he end could very well be the issue you're having. | 
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