btw, whats ur pump's output? in litre/hr
for weipro, theres a knot at the 90 degree output pipe, you can try tuning that and see if the thing improves. and see whether if there is alot of bubbles forming by visual comparison from the past.
the air intake portion might be choked. washed that part with warm/hot water.
beside, if your fish/coral are not doing well, do water change. its high chance its your water quality problem and not just attributed by skimmer problem.
test your salinity as well, best with refractometer. much lower salinity will affect skimming problem.
if you are so worried about your lifestock's health, just do a major water change first.