One last shot you might take. Soak some peas in garlic juice, and introduce them into the tank. It's another constipation remedy. Peas often help the digestive system in fish when they get constipated, and the garlic is both an appetite stimulant, and helps treat digestive tract parasites. Don't start fretting about your plec starving to death yet. A healthy pleco can go without a meal for 2 weeks without damaging his body.