Electric Blue Hap with irritated vent?

One of my 3 male Electric Blue Haps has been spending a lot of time near the bottom of the tank, and his vent looks pretty irritated, but he's still eating fine. All 3 fish are in a 55 gallon tank, and I just did a large water change. He has been like this for a few weeks. What do you think it could be?
 
Try dosing with unscented Epsom salt, 1 Tbsp per 10G tank water. Premix the Epsom salt in some tank water prior to adding to the aquarium.
 
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It should be just fine. Epsom salt is often used to raise hardness in a Malawi set up but is also used as both a mild laxative and swelling reducer.

What other species of fish are in this tank?
 
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