On my belt drive, the pump is bolted to adjustable mounts/feet. If i slacken off the bolts, i can push the pump back to tighten the belt, or forward to slacken it off.
I have also got threaded rods either side of my pump with nuts on it, if i tighten the nuts either side, the pump slides forward/back slightly.
You will probably have something similar where you can slacken the bolts of the pump and move it back to tighten the belt.