Waalaikumussalam Warahmatullah Wabarakatuhu,
Brother Infaq in general means "spending". In the Quran it refers to spending your wealth for the pleasure of Allah(s.w.t.). I don't know the Fiqhi(i.e. technical) relation between Infaq and Zakat, Sadaqah, etc. But in general, Infaq is any kind of spending done for Allah. In the Quran Allah(s.w.t.) has differentiated between Zakat and Infaaq. While Zakat is Farz, any Nafl amount of money that one spends for Allah by giving it to the poor, widows, orphans, Masjid, for preparation of Jihad, etc. comes under Infaq.
If you can understand Urdu, the Dr. Israr Ahmed's lecture about the concept of piety (Naiki ka tasaffur) in Islam is an excellent source of very important deeni knowledge, and it also convers the concept of Infaq (if I remember correctly).
Here's the link:
http://www.youtube.com/results?searc...rch_type=&aq=f
As for specifying 5% of income for masjid and calling it Infaq, then this is very wrong because there is no specification about Infaq. You may give any amount you wish(above the Zakat) to any poor person or orphan or the Masjid, etc. and it will be infaq. As such, Sadaqah is a kind of Infaq.
As for the explanation about "gutting the fish", i have never heard such an explanation, and very much doubt it. Infaq is similar to Nafl Salah, except that its "Nafl Zakat", if you can call it that. The purpose of Nafl Salah is to get near to Allah(s.w.t.) and to increase the weight of your deeds. Infaq serves the same purpose.
Wassalam