07-21-2012, 03:57 PM
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Sometimes substrate fish must be carefully selected. While it does not apply to corys, most others are territorial in varying degrees and this can manifest itself in pushing or more seriously in attacking.
On the loach, unless you can get a group of five, I would return it. Loach are highly social fish, as much or more than corys, and a group is essential. Lack of this interaction can make any loach aggressive, so this may be part of the problem too. And as someone mentioned, they can nip fins of some upper fish; this varies with species, but if you read our profile of the Yoyo loach [click the shaded name] it does mention this species fin nipping long-finned fish.
While the above applies, it could have been something else that initially killed the corys. The peppered cory is fairly robust though, but still be careful in acclimating them.