Do you have a 'favorite part' of the hobby? Reading about, searching for and buying healthy specimens? Tooling around with the lighting, plumbing, filtration on your system? For me it's got to be aquascaping.
Every marine aquarium benefits from aquascaping; esthetically and functionally. Yes, it sure looks good when done right; but a well decorated system does more than please the eye. Like a room with or without furnishings, aquascaping makes the overall environment better for your livestock. Enhanced biological cycling from increased surface area and added buffering capacity are just the beginning. Breaking up the physical space solves many social and psychological problems, providing hiding spaces from bullies (including you), and granting your fishes and invertebrates a sense of place.
When should you be thinking about what type of tank (size, shape), filtration, lighting, etc. you want in your home, office? After you've decided what types of life, and/or habitat you intend to fabricate. Tangs like high relief and open swimming spaces; nurse sharks require large empty bottom plains...
Ideas: abound for what you might do. One avenue to take is to limit yourself to what materials you have on hand and can readily acquire. Much more fun can be had by looking through fish and diving magazines, watching an underwater video, going on a snorkeling trip, sketching a Martian landscape and trying to duplicate it... Your arrangement can be "biotopic", an attempt at replicating a slice of nature; or fanciful. It's your artistic creation, and you can always add/subtract, or re-do it entirely.
Modeling: A useful approach is to make a grid, cardboard bottom and background, system-size and practice stacking, arranging before putting it all in the tank. This is your big chance to check aggregations for stability, maybe joining them with silicone rubber (expressly intended for aquarium use), special-purpose epoxy and "reef" cements. Make sure the assemblies will fit through the cut-outs in your tank top if you're an acrylic tank user.
Types of aquascaping "building-blocks" most commonly used are coral and shell skeletons, ancient coral rock (the off-white chalky kind), living live rock and the Noah's Ark collection of non-motile invertebrates; as well as a huge assortment of plastic fiberglass, epoxy et al. faux material.
Be extremely leery of putting just anything you've picked up in your system, including 'treasures' from seashore excursions, or freshwater decorations. Everything that goes in must either be chemically inert (non-reactive) or, if soluble, benefit the water chemistry. Be suspicious of even (gulp) what you see offered at XYZ Fish Store; some petrified woods, volcanic and other "rocks" are not safe for marine use. See the next section on testing and cleaning for all intended decor.
After setting up the system; adding salt, water substrate, all the gear's up and running. Before introducing any livestock.
Consider cleaning; how are you going to get around, underneath the decor to remove detritus, algae? Leave open space around the entire inside perimeter.
Beware of dead spots; provide for total flow-through circulation by way of pumps, airstones.
How about concealing tubing, heaters, probes, all the other doo-dads? Aggregations must be placed to be stable and removable. Watch out for underminers; you know who they are. Damsels, most crustaceans and more might be re-incarnated miners; they seem determined to topple your aquascaping.
Your tank might have come with an integral background; or you could purchase a commercial unit, but how about something more unique? Like a seafood restaurant menu?
Most aquarists opt for something neutral; blue paint, paper, foil, felt, cork... Don't use anything reflective; the mirror images drive some fishes and invertebrates bonkers.
If there were time and space I'd relate some of my favorite background stories. I have been fortunate to travel around a good part of the planet, visiting private and Public aquaria everywhere. Some of the dioramas in and outside of systems I've seen are fantastic. Whole caves and reefs made fiberglass, resins, epoxy paints. It is amazing to me how much can be done with simple tools and materials; and inspiration.
Outside The Tank:
Would you like the area surrounding your system to blend in theme, color, shape-wise or alternatively make a definitive statement? The point is that contemplation should be given to what's going on in your overall room scheme when planning a marine system and it's decor elements. For some novel ideas and attitudes you're referred to Hemdal (1987). He likes potted plants, bonsai, the usual prints and bookshelves, plus some novelties.
Amidst all your planning, execution, and leisurely recollection keep in mind that aquascaping is much more art than science; it is not a product or destination, but a growing process. Not completely satisfied with what you've got? No worries, go get additional ideas, materials and re-create it.
Baugh, Tom. 1985. Modular habitats. One of many advantages of living in the present is the plethora of materials that can be used in the construction of simulated environments. FAMA 3/85.
Carlson, Jeffrey. 1988. Beginning the marine aquarium (it's not just a fish tank anymore), part VII- Decorating or reefscaping your tank. Marine Fish Monthly 3(7):88.
Hemdal, jay. 1987. Decorating around the aquarium. FAMA 11/87.
Liebetrau, Sue. 1979. Aquarium interior decorating. FAMA 2/79.
McDonnell, Bob. 1984. Tank decor: who needs it? FAMA 9/84.