That's weird, did Rowaphos change or are you sure you're talking about Rowaphos and not Phosban? Because my instructions emphasize to not let it dry out, i.e., keep it moist, and there's no mention of a minimum of 250ml, in fact, the instructions recommend 100ml for the size of water volume I'm at.
Nevertheless I am still running 1/2 of the lowest dose but I thought I read somewhere that it's bad to go full strength right away because you can shock your SPS. When I first ran Rowaphos it did shock my SPS and I had losses so I was looking to avoid making that mistake again.
Anyhoo though if it can take 48 hours I'll recheck tonight (since tonight is 48 hours) and see. If it's still outputting no reduction I'll swap it out and go full strength and see.
My media is at least a year old so it's possible it's no good. Short shelf life maybe? I wouldn't rule it out, I'll try retesting and adding more if need be, see where things go.