Awesome that you rescued him/her c:
I rescued a pleco in similar conditions, it was in a 30 gallon but was 18 inches long. Poor thing just stayed curled up in the corner since there wasnt room to move. It had been in that tank for over 5 years, pretty sure never cleaned.
I had my brother wrestle him down, there was no way I could have done it LOL. There was also a bala shark in there that kept jumping up and hitting him in the face XD
I dont have a tank thats big enough for a pleco (boo apartments!) so I gave him to my lfs whose owner I'm friends with. The pleco lives in a large pond now :D