Probably not really an issue with you only changing 25%, but if your water has Chloramine instead of chlorine, the API conditioner will bind up the chlorine, but leaves the amine which is ammonia. Other conditioners will take care of both chlorine and ammonia, and other toxic chemicals/hard metals.

Good luck with your corys. Hopefully they just want more food options though. Remember if you are feeding flake food to pinch some then dip your fingers in the tank to moisten it so it will sink to the bottom. Mine typically scavange for blackworms that "escape" the feeding trough and then try to bury in my gravel.

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--->Pictures of my 75g Amazon tank<----