Coralvue sent me the wrong ballast this week. I was supposed to have gotten a 400w but they sent me a 250w. I talked to David, who is also the owner, and he told me until they send me the correct ballast that I could use the 250w one. He said that it would not have a problem firing up my 400w bulb. I have not tried and probably won't since the correct ballast will be here this week but just thought I would share with you what the owner of Coralvue told me.