Fluval Stratum Question

[typhoon]

hi, ive just bought 2 4kg bags of this and was wondering if its fine to put a whole bag in with no problems? tank is already running and stocked with discus
Tank is a rio 400

best way to add it?

possible problems?

http/www.ebay.co.uk/itm/Fluval-Flora-Plant-Stratum-4KG-Laterite-Substrate-Food-/330567224005?pt=UK_Pet_Supplies_Fish&hash=item4cf7578ac5