Gold Fish and Community Tank Fish
It doesn't matter if you're new to the pet fish world or an old hat starting up a new tank, the biggest decision you need to make is which kind of fish to keep. This is a hub about freshwater fish. I will include a link to a few places you can find info on salt water aquariums at the bottom. Both fresh and salt water tanks have lots of colorful, beautiful fish to choose from. For beginners, I suggest a freshwater tank since they deal with less chemistry and are easier to maintain.
If you choose to start a freshwater tank, the next important thing to think about is what fish you would like to keep in it. There are a number of successful groupings you may want to try including community tanks, goldfish tanks, species specific tanks or cichlid tanks. Community tanks have the advantage of being diverse, but you have to be careful to keep aggressive fish out of them as they can devastate a tank. Goldfish tanks can be a bit more aggressive, but they really do best with their own kind, especially if they are fancy goldfish like Black Moores or Telescope Eyes. Species specific tanks are good for breeding fish like Guppies, or schooling fish like Zebra Danio's or Neon Tetras. Cichlid tanks are highly aggressive and should not be combined with the other types.
Same 30 Gallon, Different Populations
Setting Up The Tank
A short note about setting up a tank. Goldfish and other fish that grow to large size usually need about 10 gallons of water per fish. Other fish typically need about one gallon per inch of fish. When you are choosing a tank and fish, it's good to keep those guidelines in mind so that you don't overstock your tank.
Also, it is necessary to allow a tank to "cycle" when it is first set up. I could cite a whole bunch of fancy speech at your about why it's needed, but all you really need to know is that your tank will take about a month to be safe for your favorite pet. You can go about cycling in two ways- one is to let the tank sit empty for a month. The other is to buy a few inexpensive, hardy fish and allow them to adjust the water. Feeder goldfish and corydoras catfish are both good candidates for tank cycling. I have found that both are hardy enough to withstand cycling water and are inexpensive enough to be easily replaced. Either way you do it, don't put your most prized, beautiful, exotic fish into the tank for at least a month after you set it up.
Also, all tanks do better with some "bottom feeder" or "tank cleaner" fish. These fish (or invertebrates) keep tank and walls cleaner so that you have to change water/scrape out the tank less frequently. Good fish to buy to keep the tank clean include corydoras catfish, plecostomas, apple snails and chinese algae eaters. A word of caution on the pleco's and chinese algae eaters: the pleco's are delicate and easily die in poor water conditions. They also are very nocturnal and it's not uncommon for them to hide all day. You should always provide a place for them to hide so that they feel safe during the day. Chinese algae eaters can be very aggressive. I'd never seen that behavior until I kept one in my guppy tank two years back and it began shredding their fins. Later I looked them up online and saw that they were in fact listed as semi-aggressive and did grow much larger than I originally believed. They should only be kept with other slightly aggressive fish and only in larger tanks.
PookPook: A Healthy Black Moor Goldfish
Friends or Enemies?
The key to choosing fish is knowing how they will get along. There's nothing worse than watching your beloved fish chase each other to death. Here are some common fish and some recommended tank mates:
Common Goldfish- This is a semi-aggressive fish. They can be kept well with carp (read, catfish-like goldfish), other goldfish, and bottom feeders. Catfish and pleco's go just fine with common goldfish. I have heard of keeping Gourami's and other semi-aggressive fish with goldfish, though I have not tried it myself. Provided they are all about the same size that should work out just fine.
Fancy Goldfish- This includes fish such as the Black Moor and Telescope Eye goldfish. I really recommend that these fish be kept by themselves. I have seen a lot of fin and eye injuries in these fish if they are kept with anything but other fancy's and a few bottom feeders. Catfish and Pleco's seem to do okay with them, but anything fast or aggressive just tear up these fish unmercifully.
Beta Fish- These aggressive fish can do well in much smaller tanks than most. They have a special organ that allows them to get oxygen from the air as well as the water. This means that a tank only holding a half gallon to a gallon of water would be acceptable for a lone beta. Many creative, fun types of small tanks are made for betas and can be bought for a reasonable price. If you do purchase a small beta tank, I do recommend getting one with a filter so that cleaning can be less frequent. A snail in such a small tank is also a good idea because the beta will not harm it and the snail will help keep the tank clean. Also, betas can do well in a community tank with fast, small fish as long as they don't have any flowing fins to shred. Good tank mates include neon tetras and zebra danios.
Tetra's, Barbs, Danio's and Other Schooling Fish- I do not include guppies or swordtails in this category because they are livebearers and slightly different. Good examples for this category are Zebra Danio's, Bala Sharks, Ghost Glass Fish, Neon Tetras, Glowlight Tetras, Rosie Barbs, and Red Eye Tetras. There are plenty of other Tetras and Barbs that fit here as well. These fish are very good to keep in a community tank and some can do okay with normal goldfish, though I don't recommend that they be kept with fancy goldfish since the fancies often have sight issues and long fins that can get torn up by fast little fish who can see better and swim faster. Also, these fish do the best when kept in groups of five. At the very least groups of three are acceptable but their remarkable schooling behaviors are best observed when in larger groups. A few barbs can be mildly aggressive so try to keep them with fish of the same or larger size.
Loaches and Knife Fish- These fish are very dissimilar though their appearance is somewhat close. Loaches can come in a variety of shapes and sizes. Some are worm-like while others can resemble freshwater clownfish. They are generally passive, quiet fish that do well in a community tank. Knife Fish, by contrast, vary widely in their temperament. A few can be passive but most are extremely aggressive and will pretty much eat anything smaller than it in the tank. I've seen a friend's tank decimated by an aggressive knife fish.
Angels, Paradise Fish and Gouramis- Angels and Gouramis are semi-aggressive fish that do alright in community tanks with other fish their own size. Some smaller, fast fish such as streamlined tetras or danios can do well with them also because they can move out of reach quickly. Paradise Fish are similar but they have flowing fins that can easily be damaged by fish of the same aggressiveness level. They can be kept with members of their own species and some bottom feeders.
Cichlids- I have less experience with cichlids and thus have less information on them. What I do know about them is that they are hyper aggressive and do not make good community tank members. They should only be kept in a strictly cichlid tank. However there is a large variety of them, so if you want a diverse tank you can still keep them just make sure that you keep them with others of the same type.
Guppies and other livebearers- I have had great success with guppies in a species specific tank. They breed like crazy in a slightly dirty tank. Give the babies a few places to hide and soon a three fish tank can turn into two-dozen fish tank. Fancy guppies come in dozens of colors and are quite beautiful to watch and raise. Swordtails and some other livebearers can be hyper sensitive to water quality. If you are very careful to keep the water clean and at just the right temperature, these fish can be quite beautiful. They are fine to be kept in the same environment with the Tetras and Barbs.