hi iv decided i need Purigen my water is a bit cloudy/brown. so ive read up about it but, im not realy sure how much i need.seachems web site says "Each 1 L treats up to 4,000 L " other web sites say you need more. their actually selling it so i guess they would.so going buy the manufactures recommendation for my 750l tank i would only need 250ml is this right. im using a sump filter so i was just going to put it in there on top of the filter pads. thanks for any help im slowly getting there.