Why are my loaches bullying my catfish?
Although catfishes are a very broad group and include some species that you wouldn't want to share a tank with, most commonly-kept community species are either peaceful shoalers like Corydoras or rather reclusive algae eaters. In contrast, most loaches are rather like their relatives the barbs and treat everyone in the same (sometimes obnoxious) way. Like barbs, they naturally spend time in groups where a constant jostling for resources and status seems the norm and it's true to say that when kept in larger numbers they make better neighbours.
Some species are best kept away from peaceful bottom-dwelling fishes and these can be a real challenge to house with anything that isn't equally grumpy. Tiger (Syncrossus spp.) and Skunk loaches (Yasuhikotakia morleti) are frequently seen examples of fish that are perhaps avoided by most aquarium keepers.