The present invention relates to a substrate sheet for treating aquarium or pond water. The substrate sheet comprises a water-soluble carrier film and a treatment agent. Suitably, the treatment agent is a de-chlorination agent, such as sodium thiosulphate, and/or a redox reducer. The water-soluble film can comprise a strip forming polymer and/or a plasticiser. Examples of suitable strip forming polymers include starch, cellulose or dextrin derivatives, methylcellulose, ethyl-cellulose, hydroxypropyl-cellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose (HPMC), carboxymethylcellulose (CMC) or a combination thereof. The plasticiser can include polyvinyl alcohol, glycerol, propylene glycol, low molecular weight polyethylene glycols, dimethyl phthalate, diethyl phthalate, dibutyl phthalate, citrate derivatives, triacetin, castor oil or a combination thereof. The water-soluble film can be coated and/or impregnated with the treatment agent. Preferably, the sheet is in unit dose form.
