First off, I would say you have two options for how to go about doing this...
Option #1 is what Devilishturtles just wrote above: decide upon a general "type" of tank and then once you have decided that, research what types of fish fit the profile.
Option #2 is to look online, or just go to a few local fish stores (
lfs) and look at what they have. If there is one type of fish you just fall in love with, write down its name, then go home and research what other fish make good tankmates for it.
Among the things you will have to consider with tank mates are...
-- AGGRESSION. Aggressive fish will tend to bully timid ones, leading to death either directly or indirectly (a stressed fish eventually becomes a diseased fish, etc.)
-- SIZE. Not necessarily related to the above, though it often is. Sometimes non-aggressive fish can become...um...unusally aggressive when they are in a tank with other fish small enough to fit in their mouths. 8O
-- SPEED. Some fish are very active, bordering on hyperactive, others very slow moving. A very large but slow moving fish can actually get freaked out if it is in a tank with a school of smaller but very hyper fish (like zebra danios). The net result is the same as a fish that gets picked on; it gets stressed and is more likely to develop disease. Or, perhaps, all it does is stay in the plants/caves and hides all day...and what's the point in having a beautiful fish if it never comes out to be seen?
-- STRATA. Some fish like the top part of the water, some the middle, some the bottom. When planning fish, be sure that you aren't getting a whole ton of fish who all like the same strata, but get a nice mix so that the whole tank looks "full" while not being overfull. Also, having at least one type of bottom feeder (whether catfish, or something like ghost shrimp, whatever) to help clean up food flakes on the bottom of the tank is a big help. It doesn't replace regular maintenance, but it helps keep things clean between your regular gravel vacuuming.
-- NUMBERS & SEXES. Some fish really need to be in schools in order to thrive. (Note: schools usually means 6+ of the same species). It is really not healthy to have only 1 or 2 or 3 fish of a species if it is a species that needs to be in a school. It's just not fair to the fish. Other fish do best in pairs or threesomes. Often sex is important; for many species, a male/female pair will do fine but having two males will lead to major territorial disputes or even fighting to the death. (Bettas are the most common culprit here, though many gouramis display this male/male aggression as well.) For other fish, having a male/female pair puts too much stress on the females, and it is really necessary to have 2 or 3 females for every male in order to spread out his...um..."attention." Many livebearers (platies, mollies, swortails, etc.) fall into this category. Also, I have rainbowfish, and though I could keep a group of all males, I read about them and discovered that males will not really show their best/brightest colors unless there are also females present in the tank--so even though the females themselves are not as attractive, I got them in order to bring out the best colors in the males.
-- FIN NIPPING. Some fish are notorious for being fin nippers (e.g. tiger barbs), and should not be put in the same tank with fish having long, flowing fins.
As for your other questions, I would suggest plastic plants at first, just because they are far simpler to maintain. With real plants, you need to monitor your lighting (lighting that is enough to make the aquarium look pretty may not be powerful enough to give your plants the
UV light they need), and your
CO2 levels. Plus whenver you bring live plants into your tank, you risk bringing in snails or other invited guests who come along with the plants. So start with plastic, and if 6 months or a year down the road you decide you want live ones, that will give you a new twist to keep yourself occupied.
And no matter what, be sure you have some decorations that will make caves/hiding places. Even very outgoing fish can sometimes feel uneasy when first moved to a new tank; having hiding places eases the stress of that transition. And some fish regularly need caves. So whether you buy pre-made decorations in a cave-like shape, or make your own out of wood, rocks, etc., be sure some caves are there.
Sorry if this was too long. Hope it helps some!
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