Your pleco is probably your plant eater. In general, most experienced plant keepers will not keep a pleco in their tanks, as they will literally turn your tank into one giant salad bar. If you want a pleco look for more plant friendly kinds... I've had a clown pleco in my 70 gallon tank for more than a year, and my plants are all doing very well. The bala shark might be nibbling on your plants, since they are omnivorous... as are most of your other fish, but they shouldn't be posing a problem.
On another note, I hope you will reconsider keeping some of those fish in your tank. Some of those fish should not be kept in a tank of that size.
The common pleco (which I assume yours to be), can grow to be quite large... I've seen many that were at least a foot or longer. Fully grown plecos require tanks that are much larger than yours.
Bala sharks also require a much larger tank then what you currently have. Bala sharks like to swim in groups... I would suggest somewhere along the lines of at least 4-5 in a group. Bala sharks can also grow to be quite large... somewhere along the lines of about about 12inches... though most aquarium kept ones don't grow that large.
Regardless I personally don't think bala sharks should be sold to the regular keeper. They need much more swimming room than a normal keeper can usually provide, and on top of that they usually do better in groups.
HTH
-Richer
