Ok. Let's try 30% water changes every week for starters. Once a month is for walmart. If you're running a UGF, try taking the water you remove off the bottom instead of the top. UGF can be lower current, and poison sinks. Last but not least, check your substrate for pockets of toxic anerobic bacteria, and the lethal gas they produce. You can do this by running your fingers through the substrate. If you get bubbles, you got toxic build up of decaying waste materials breaking down in a zero oxygen environment. Your bottom feeders disturb the substrate digging for tasty food and get a blast of poison gas right in the face and gills. This can produce sickness or even death within a very short period of time. To remove it, sift, clean, or even remove and wash your substrate. Last is a desperation measure, that's your biofilter with a UGF set up. At the least, if you get bubbles, remove your fish from the tank (put em in a bucket of tank water) sift all the bubbles out with your fingers, then do a big big water change, and replace your fish. That'll take care of the immedate threat, and you can keep sifting to keep the gas from building up.
On one more note. Algae alone is'nt enough to keep a pleco fed. Some of the non-common species require wood, meat, fresh veggies. All depends on the species. If you got him at petsmart, he's most likely a common, chocolate, bristlenose, or gold nugget. All of these like algae wafer (wardley makes a great product) and shrimp pellets (omega one is tops there).
Common or chocolate will outgrow your tank rather quickly when fed. Gold nuggets will outgrow it eventually but may take years.
