Does My Fish Need A Friend?
When keeping fish, it is hard to know whether or not they are lonely. This is a quick guide to tell you whether or not you should get your fish a friend.
Whether your fish should have a friend depends on two factors. Firstly, the amount of space you have in your tank, and secondly, the type of fish you have.
The general rule of thumb is that you should have about one inch of fish to four gallons of water. If you have a two inch goldfish, and an 8 gallon tank, then no, you cannot get anymore fish. (If you have a two inch goldfish and an 8 gallon tank, you need a bigger tank, because a healthy fish will grow longer than two inches and you don't want to stunt and kill your little fish friend.)
Say you have a decent sized tank, however, like a 20 gallon, then you definitely have room for more than one goldfish – you have room for two. If you want to keep more than two fish, then I would recommend that you heat the tank and go for small tropical fish, which are less messy. Personally, I don't recommend buying any fish tank under 20 gallons with the exception of tanks for Siamese Fighting Fish, or Betta Splendens, who will do very well alone in a 5 gallon tank.
If you have the room, then you need to think about what sort of fish you already have. Some fish are naturally solitary. Betta fish, for instance, do much better on their own than they do with other fish, in fact, unless you are breeding, betta fish should not be kept with other betta fish.
Other fish, like neons and danios, are schooling fish and should never be kept alone, in fact, you should always keep at least six of them so that they feel safe and can swim together. Mollies and guppies also do well in groups, but be careful to keep an all girl tank unless you want babies popping out all over the place, guppies especially breed like crazy, and it is easy to have a tank overrun with baby guppies in just a few months if you are not careful.
It's impossible to go through every species of fish here and tell you whether or not you should get a friend for them, but a little bit of research on your part can quickly tell you if your fish is a sociable, community fish that needs friends, or if it is a loner that will be quite happy with its own space. Most small tropical fish will happily live in communities, however fighting fish and many cichlids can be quite aggressive, so you need to be careful when you add new fish to their tank.