Did you catch these? What kind of spices are you incorporating? Were they marinated first?
I cooked up some canal plecos a few years back and used a simple recipe I saw used in Panama.
Per pleco (gutted and cut behind the gut at an angle forward through the body):
1 shredded mango
salt & pepper to taste
Place the cleaned plec in foil, cover with shredded mango, S&P, and pour over some of the mango's juice). Wrap and place in oven. Bake at 350F for about 25 minutes. Check if additional time is needed.
The meat went in dark but, whitened up as it cooked. Came out flaky, firm, white, and mellow-tasting. (more mellow than tilapia and better flake.)
Cooked the next ones the same way but added a couple of shakes of zesty italian dressing to the mix. Came out awesome.