a couple thoughts:
a refuge does best when it has a slow flow...around 10gph or so....what is your gph through the refugium?
I personally don't have a big pump throttled back by a ball valve because the pumps generally don't last nearly as long that way....I would either assume that you have it held back alot or open with a high gph running through your refugium. The way that I solved both of those issues is illustrated in the refugium diagram below.
just my thoughts,
john