I’d suggest getting the ‘Photographer’s Market’ book (
http://www.photographersmarket.com/). It’s published annually and is a listing of photo buyers and what type of images they need and how to submit them. It won’t help you a ton if you only have photos of fish or aquariums, but it will give you a really good general idea of how the process works for a number of different segments of the photo buying industry.
Another book that I’ve found really useful is ‘Pricing Photography: The Complete Guide to Assignment & Stock Prices’. It is pretty much what the title says.
Most editorial (magazines) buyers usually set the rates they pay so you won’t have very much to do with the pricing.
Find the ‘photography guidelines’ for the publications you are interested in selling images to. They will tell you exactly what the magazine is looking for, how to contact, how to submit and (sometimes) how much they pay. (TFH’s are here…
http://www.tfhmagazine.com/inside-tfh/photo-submission-guidelines/) Then you start building those relationships that Paradise mentions (you can’t avoid it).
Another avenue is stock photography. Basically, you provide your images to an agency (library) and they try to sell / license the photos to their extensive list of clients and keep a (substantial) percentage of each sale.
It is getting tougher and tougher to make a decent living in any aspect of photography, especially if you are specializing in something that has limited market appeal like aquarium photography. Part of what is making it so hard to make a living is MicroStock.
MicroStock might me an option for you if you are not too serious about making money and just want to get you images out there. MicroStock operates similarly to Stock Photography but is geared more to hobbyist photographers. They license photos at very, very low rates (usually on a royalty free basis) and pay the Photographers a small portion of those very low rates. You will NOT make very much money with MicroStock but if you are just wanting to mess around with your images and be a part of another photography community (all of the micro stock agent sites have helpful forums and informational resources).
Good luck!!