What is wrong with my male guppy?

Question

I have 3 male guppies (along with a small plecostomus) in a 10 gallon tank. Today, I noticed one of the guppies was acting strange. He swims sporadically against the glass sides and then just drops to the bottom of the tank like he's dead. He even moves around with the current from the filter, ending up resting on his side or even upside down. Sometimes he doesn't even land on the ground and lays like this on the plants in the aquarium. His tail and fins seem rotted and his mouth looks red. He just looks sick in general. The other 3 fish are perfectly normal, but I quarantined him just in case. What could be wrong with him and what could I do to fix it?

Answer 1

The rotted fins are a sign of fin rot, usually caused by water quality problems. Test your tank water, plecos can foul it up pretty quick.

It could be a variety of things, the red marks suggest a possible infection, which would lead to a low immune system and more susceptability to water quality issues.

He sounds beyond saving and I would put him out of his misery. Watch the surviving fish carefully for other signs such as more red markings. Keep up on your weekly partial water changes. Fingers crossed this guppy is an isolated incident.