Our large pleco's fins on the back and tail look like they are eroding. Is this normal or a problem and if a problem, what can we do? We have mollies, two other small plecos, danios, etc. in a 55 gallon tank. We put summer squash or zuchinni in the tank for the plecos about once a week. He also eats the fish food.

sounds like fin and tail rot. If it is there is meds that can clear it up. It could also be that your water quality isn't that great. Sometimes cleaning the gravel and changing water can clear it up. I'd try the water changes/gravel cleaning first... meds can be stressful for your fish.

Yes, it could be fin rot. I suggest doing a 50% water change. Add aquarium salt at a tablespoon per 5 gallons and maybe some antibiotics like Maracyn I and II by Mardel. If the fin injuries seem like the fin is eroding, that's probably fin rot. Fin rot is common with poor water quality. Test your pH, ammonia, and nitrite levels to see where they are even if the tank looks clean. Be sure to remove any uneaten foods you put in as squash can be messy and make the water less pristine. How big are your plecos? If they're big (over a foot), keeping the water really clean can be a challenge. If the injuries appear more to be tears than disintegration, then I would consider that something mechanical in the tank (filter intake, cave, etc.) or other fish (plecos, mollies, etc.) could have damaged or bit the fins.