Help added a pletco to my tank & things started dying ..

[CoolCrab]

Hey guys I just started picking up the hobby a few months ago I believe I have a 50-75 gallon freshwater tank that use to be a salt water , My wife had it for a while and I started gaining interest my water is tap water, I have a good filtration system and the water tests are perfect it’s also between 76 - 79 degrees F. I had my red Thai devil crab for a while. My wife surprised me with this pleco for my birthday, I wouldn’t have got it myself but it was a present from wifey .. So I have other fish and they get along well except my oscars. My question is will this pleco eat my crab he’s been acting strange and I had two other crabs of the same species and they both died two days after I added this pletco and a green terra which is about (3-4inchs) and she’s awesome ! If anyone has any info I would greatly appreciate it because now I’m stuck watching my tank because I love this crab and appreciate the pletco it just doesn’t feel right I found the two crabs covered in poop also the crab is now cornered which is common but he looks like he’s more defense then usual also the pleco took a interest in that area .. thank you all!

P.S additional info: Also when I added the pleco & other fish I also moved around the rocks so the big oscars would settle with them better and I had one more green terra I found him in the little round log dead way before the crabs.I feed the pleco algae wafers from terra as well as some veggies once a week I had the crab in a divider in my tank but it was getting hard to clean and building up sludge