Its difficult to say but he's definatly either an albino sailfin or common plec but i am more leaning towards sailfin becuase of his shape, true albino plecs don't look anything like those and are very rare.
If your plecs a sailfin, he's gonna grow to 2ft long and need a 125gal tank.
If he's a common he'll grow to about 12inchs+ long, and need at least a 55gal tank.
Both will grow an inch a month and need an endless suply of algae wafers as they ahve big apetites, you should also do somthing about your current common plec.