The first thing to have in mind is your tank size. This is a very important element in fish choice because many of the small fish you see at the fish store can easily grow to up to two feet! It is also very important not to crowd your tank. In other words, give the fish some space! The main rule is one inch of fish per gallon, so a one inch fish gets one gallon of water, a two inch fish gets two gallons, and so on.... Just to be on the safe side, I would have about one or two gallons to spare. (i.e: 8 fish in a 10 gallon tank) (see http://freshaquarium.about.com/cs/beginnerinfo/a/fishcalc.htm for more info)
Rule number: READ THE LABELS!!! You know how at the fish store they have little labels/signs for each fish type (often include the price)? They can provide you with the lifespan, maximum size, behavior, what they eat, and possible tank-mates. Which leads me to my next two rules...
Make sure your fish won't outgrow your tank! I have many this mistake more than once and it only leads to an unhappy fish or a sad goodbye (if you had to sell them). This information can often be found on the internet,in books, or simply on the fish's label at the store (you can also find some of this information on the 'Fish Species Info' tab. ;) Believe me, you will have much less worry if you follow this rule :)
Next: Make sure the other fish in your tank will get along with the new guy. This is called fish compatibility. Putting the wrong fish can lead to fish injury or death. (I have also made this mistake with the purchase of Aphrodite. Mixed Fruit Tetras and guppies are not compatible! Mixed fruit tetras have a bad habit of nipping off long tails) if you need help finding how who gets along with who, check out these websites: http://tropicalfishandaquariums.com/Compatibility/index.php and http://www.ratemyfishtank.com/freshwater_compatibility_tool.php
While you're at the fish store, pay close attention to any dead or diseased fish. This may be a sign that there is a parasite or bacteria in the water and buying a fish from that tank may carry the disease to your tank.
You should also learn how to sex your fish. (Tell what gender it is) This is important because many people accidentally mix different gendered fish and end up with more fish than they can handle,which causes stress to both them and the fish! Only mix different genders of fish when you are purposely breeding them,and if you are, be prepared!
Keep in mind, if your fish likes to be in a school (multiple fish) you should provide them with a school
! (I.e: Don't just buy one neon tetra, buy at least three or more!)
I do NOT recommend buying or transporting during cold winters or temperatures under about 30 or 40 degrees! It can be life-threatening to your fish! (Yes, even for just a couple seconds... if you absolutely must, try to keep them warm with your jacket, etc.)
When you get to your house with your new fish, be sure to acclimate them to the new temperature and pH by setting the bag (still tied up with new fish in it) in the water for about 20 minutes. Also,don't mix the water in the bag with your water, instead, open the bag, use the net to transfer fish to your tank, and dump bag water into the sink. It helps to turn off the light for the first couple of hours after they get there to alleviate stress. It is normal for new fish (especially small ones) to hide for a little while after you get them.
And now for my top three personal recommendations...
Guppies: They are very pretty and can be found in almost any fish store!
Neon tetras: They're very cute and fun to watch when in schools [of fish].
Any algae eater: (Such as a snail or corydoras catfish) They keep gravel, decor, plants, and your tank's glass sparkling clean! (Plecos are great if you have room for them)
If you have any other questions, feel free to leave them in a comment or ask a fish expert at your local pet store! ;) I'll be posting more helpful tips and tricks about every month, most will probably be a lot shorter than this, but hey- I'm giving you a lot to start off.
