Where did you find a copy of Spotte's Captive Seawater Fishes? That book is like $300 on Amazon, IIRC. I have a copy myself, purchased many years ago, before the price got inflated. If you have $500 to drop, I would suggest Stoskopf's "Fish Medicine".
For free, try looking online for a pdf download of "the Elasmobranch Husbandy Manual" - the topic is sharks and rays, but it covers a lot of additional advanced material. I think it is available through multiple bit torrent sites. I normally abhor these sites, but since that manual was free to begin with, there is no copyright infringement.
Walter Adey's "Dynamic Aquaria" is another important book.
Jay Hemdal