You could use the whole 250 in a 50 gal. It will not hurt anything. The purigen will just last longer before you need to recharge it. I just put 200ml into my 2026 on my 30gal.
P.S. Over at
The Planted Tank Forum - Powered by vBulletin there is a seachem sponser area if you want to ask a seachem rep. directly. I think they check that sight and answer questions that are posted.
There are already a few questions and answers about purigen there already.
Sorry but that thread is gone. Seachem must not sponser there anymore. ugh.... What I said above I have read from a seachem rep. before. Do not be afraid to use it.