problem with Q. tank and royal pleco

[StiffMeister]

guys, a few days ago i bought a royal plec. its been in the store for 2 weeks on its own in a tank. just to be sure it isnt ill or something ive put the fish in my quarantaine tanl. but now i have a problem with this tank. i cant keep the water good for some reason. changes dont really affect so i guess the pump is just fecked. would it be wise to keep the pleco in this tank or can i just put it in the main tank. water there is good and i guess it will not be sick as its been in quarantain for 2 weeks in the shop... whats wise??