Possibly White Spot Disease (Ichthyophthirius multifiliis), commonly known as Ick or Ich. Immediately do a 50% dechlorinated water change and use a medicine for this condition. Make sure the temperature of the water is fine and that you don't cause more stress by using water with a totally different pH level.
Example of medicines that are available to treat Ick.
Since I prefer to keep smaller tanks (under 50 gallons) I've only had one Pleco. Based on my brief experience with Plecos, I can say that mine was one of the hardiest fish I have ever had the pleasure of taking care. Mine was given to me by a friend who had a neglected 55 gallon tank. No water changes for months. No heater. Many other fish died in my friend's tank, but not the Pleco. I had the Pleco for about a year. Did many water changes.. Many medication treatments, because I had some Mollies with the Pleco.. The Pleco made it through the entire time with flying colors. No problem.
On the packaging there are usually warnings about which fish need lower dosages.. For example, on the Quick Cure, for Tetras it is recommended only to use one drop for every two gallons whereas it recommends one drop per gallon for all other freshwater fish. The Maracide contains Tris (hydroxymethyl) aminomethane and Dibromohydroxymercurifluorescein(Say that ten times quicker). Quick Cure is basically a type of Formaldehyde.
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