I hope you don't mind me putting it here but I have a simple diafine question too.
Why would you buy a gallon instead of a quart size? If you buy the gallon size, can you store the powder and mix new from it? That would make sense. But if it lasts as long as people say, I don't see why you'd want a gallon of liquid unless you were using really big tanks.
I bought the quart about three years ago and it's still good. I do flood the bottle with nitrogen tho, so that probably helps, but Diafine will last a long time and from what I have read it's better seasoned. You don't use it one shot.
For the original poster: Filter the stock solutions before putting into the developing tank.