Elysia,
There are two primary manuscript purchasing methods, royalty based and work for hire. Aquarium books are most often works for hire - you are paid by the word. I generally pass on those that pay less than 12 cents / word. 10 cents is what they paid authors back in the 1950s! Most books run 20 to 45k words. Royalty-based projects are much rarer, I've only done two. There are all sorts of caveats with them - e-sales, overseas sales, etc., that cut into the percentage, but it is usually 12 to 15% for full price book sales (of the wholesale price, NOT the retail).
Bottom line - things have really changed Herbert Axelrod wrote his first basic aquarium book in the 1950's and bought a HOUSE with his earnings. I wrote a book on fish breeding in 2003 that was just as long, and I bought a CAMERA with the proceeds (grin).
That said, you also get paid for images used - and Scott would make much more money for his images than for the text. The problem is, this makes the books VERY expensive for the publisher....they prefer to use stock photos to illustrate books to save on costs.
Jay