Pleco In Trouble, Need Help

Hi everyone. i'm new to fresh fish, and have some troubles.
I had two plecos about 12 inches long. All of a sudden I found one of them dead on its side, a week later the second one started to turn over, for the past two days every time it tries to move ends up on the side and can't turn over, I have to help it. I also have cichlids and if it were some kind of virus, they would be dead first. Does anyone know what is going on to my fish?
Thank you..

You need to post your water stats:

pH / Nitrite / Nitrate / Amonia

Have you added anything new to the tank?

What species of plec was it?

Were there any external signs of infection?

we checked water yesterday and it was fine, nothing has been added to the tank. Here is how it's been in the last couple of hours, it is alive, but in this position and doesn't move, if it tries to move it ends up in the same position on the side or upside down..

and my tank is about 35- 40 gallons, and pleco is about 12 inches, the other was about 10-11.

What do you mean by fine?

20+ inches of plec in a tank that size = a lot waste.

The levels of a amonia, ph were good, a very slightly raised Nitrate. I've changed 20 % of water since.
Yesterday it looked completely dead, without movement and all the fins down, today was a bit alive, with fins up and tried to swim, but constantly needs attention, It was lucky I was home and could turn it up right everytime it needed help. It seems like it stock in that position, see picture above.
Thank you for your attention.

what do you feed it? you might want to try some shelled peas and then a varied diet of vegetables