Tetras are good, and you could get a few small corydoras cats that will stay on the bottom. Or even one or two of those dwarf frogs.

blackmoors and neon tetras would look nice in your tank

neon tetras although i dont like them they look nice.;-)

Don’t go with goldfish they will eat your guppies!!!!!!!

gold fish

you’ve got a real mix of answers in here some r great and some are horrible. first of all, if your tank is heated, you can get any community fish, you might like sunburst wag plattys, or blue mirror tetra, but there are lots of different kinds. second, do not use gold fish, they are filthy, need cold water, and get huge. third, if you don’t have a heater, i would get one they really aren’t that much trouble to deal with, you need a 50 watt, no matter what the packaging says and the fish like it at 78 degrees. and finally i would only recommend having 6 fish in that tank total including any algae eaters or cat fish you choose to have. the general rule is one inch of fish per gallon of watter when they are full grown and most of the community fish get to be about 2 inches (yes, including your guppies)

Swordtails or neon tetra should be fine. I also like to keep those small brown eels, they helps to clean the bottom of the tanks.

Sarah D is absolutely right when it comes to goldfish. ANY goldfish will need a 50 gallon tank atleast. Go to a fish store (not Wal-Mart, or giant chain pet shop) and they might half full grown fancy gold fish. They are HUGE. Comet and regular feeder goldfish even grow too large for a 10 gallon.

When you buy a fish that is in a tank too small to grow, it won’t seem to grow. But really, the internal organs are still constantly growing, but is getting squeezed tightly between the body of the fish, which won’t grow because there is no space to (tank is too small!). This is called stunting. The organs will continue to grow but the body won’t, so the organs get crushed in and the fish dies. Humans can live in a jail cell all their life as long as they have food, but you wouldn’t be happy in one.

Edit: I kinda forgot to answer the question eh? Well, chances are you have a male and female livebearer. They will be making babies like rabbits. Soon you’ll have more fish in there than you can handle. If you are still keen on getting more fish, avoid other livebearers (guppies, platies, mollies, and swordtails are common). Get some tetras instead; neon tetras are nice.

Be sure not to add goldfish of any kind. 10 gallons is too small for them, and goldfish should never be mixed with tropical. Try some cute litttle platties. They come in all kinds of colors, swordtails too. Neon tetras are fine, but there are so many types of guppies out there it’s possible to get a great variety with a guppy only tank.

Do not put a goldfish in that tank. No goldfish belongs in anything less than a 20-gallon tank.

Do you have two males or two females? If you have one of each, you really need to add another female, as that male will exhaust that single female. Also, they breed like crazy, so expect babies if you have a mix.

Is this tank cycled? Is it heated? You could add a gourami or two. They can be beautiful fish, though they may be aggressive towards your guppies. Perhaps a couple of Purple Passion Danios or Glo-lite Danios. Or you could add two corydoras cat fish (they’ll help keep the bottom of your tank clean)

Instead of fish, ever think about a frog? Look into an African dwarf frog, as they can be very personable.

You are kind of limited with such a small tank. You are better off looking into getting a bigger tank, and then your options increase tenfold.

Good luck!