I had been an active researcher until two years ago and published several papers and reviewed several papers as expected. Two years ago I took a teaching position. I teach 3 classes/semester but most classes meet 4 or 5 times a week, so it actually feels like 4 classes. I also do extensive work with undergraduates. I do some research, but it is very minimal (corresponding to the high teaching load).
Due to my previous research past I am constantly getting requests to review papers (3/month). Due to my change in work duties, I have to decline many of them (but I do review some papers).
My question is: Should I explain that I am declining due to my change in work duties? What about if it is the same editor or journal that keeps sending me papers to review? How do I best/ethically proceed without hurting my chances to publish the few papers I plan to submit?
Additional information to clarify: I am not too worried about my chances to publish (my current position requires very minimal research). I am mostly worried that I will get known as a bad professional. By the way, my current institution is officially a top tier research university (and they advertise themselves as such), but my department is not (we barely have a masters program). It is very easy for colleagues to think that I am active in research unless I explicitly tell them about my current duties. It gets even more confusing since some papers take a long time to get published (after acceptance).