Sentence:  Not all fish have swim bladders, and those from certain tropical or semitropical areas (roughly 20° N and 15° S) provide the best quality <WSD>isinglass</WSD> for fining purposes. Of these fish, the threadfin family (Polynemoidia) provide excellent quality. In order to gain a desirable fining quality for a particular beer type, two or more kinds of <WSD>isinglass</WSD> may be blended. To illustrate this more precisely, "Polynemus" <WSD>isinglass</WSD> provides a fining that settles densely, whereas "Silurus" <WSD>isinglass</WSD>, from the great catfish family (Siluridae) in South America, gives a more flocculant fining but which settles less densely and consequently is more easily disturbed. 
Meaning of the word marked with <WSD> tags:  A form of gelatine obtained from the air bladder of the sturgeon and certain other fish, used as an adhesive and as a clarifying agent for wine and beer.