This has been asked many times but I see you are a 'Newbie' so here is my suggestion. Use a gravel vacuum. If you don't know what that is, it's a ridged plastic clear tube (choose one to match depth of tank) that has a flexable tube fitted to it. The idea is that the ridgid tube goes into the tank with the other end in a collecting vessel. Once you start a syphoning action keep the tube approx 1" above the surface of the sand. Then do a sort of figure of eight maintaining an inch above the sand. This will disturb the poo etc. which will then be sucked up by the syphon action. If you keep the other end in your other hand then if you accidently dip too near the sand and it begins to be sucked up, quickly place your thumb over the tube for a few seconds. This will stop the flow allowing the sand to return to the bottom of the tank. Releasing your thumb will commence the syphoning. When you have gone over all the substrate like this, keeping a close eye on the amount of water taken out,(not more than 30%) stop the flow. Then replace the water with fresh (conditioned ie. de-clorinated) and you have cleaned your substrate as well as completed a water change!
Now, i got into a bit of a spot on this forum a while back about plecs. I will word this very carefully! MOST plecs (but not all) grow very big and your tank is unsuitable for them. There are smaller ones so chose only with expert advice. Maybe have a look or post a question in the pleco forum (under catfish). My knowledge on plecs is limited to the above so cannot advise you further but there are many on this forum that will be glad to.