For a common plec thats 40cm your looking at a tank that is 72 inch (182cm) in length & 47 inch (119cm) in width as a MINIMUM having one in a 48cm tank in length is ridiculous, one way to do a comparison is imagine standing in a room where the ceiling was 1 inch taller than you and your shoulders are touching the sides of the walls all the time, each time you try to move your arms or turn around your scraping off the walls, see how comfortable you would be in those conditions and how long you would live, baring in mind a common plec can live for 10 years+
if i recall correct that example / comparison Ive used was one of the most severe punishments given to a prisoner in ww2 @ Auschwitz in Block 11.
I would take the pleco back to the store and shop else where, where the lfs will give you good information.