HI there!
Sorry you got some rude comments from several people, who seem to have not read the whole background situation behind your question. Hope the big fish were rehomed by now and they are happier in their new residence.
As for your question: What fish would work best in a 15 gal tank?
A small school (3-6)
corydoras would look sharp and help keep the tank clean (go with albino or light colored ones if your substrate is dark or dark colored corys if the substrate is light, so you can see them. My son has 3 albino ones, along with several young
goldfish (most of them white or white & orange... the goldfish will be relocated to a pond or stock tank when they outgrow their tank) in his 29 gal with
black gravel and a mixture of real and fake plants. The combo of white and orange fish looks really sharp in there. As would other very brightly colored fish.
After algae appears, I'd go with a couple of colorful snails to help keep that in check. Guppies or other livebearers are a great choice for color and there are lots of small time breeders here in the USA online with their own websites or selling on ebay or aquabid. This way you can ask the seller questions on how they were raised, the water conditions their fish like and you can ensure you get juvenile fish that are going be more adaptable to what your water conditions are. Most big fish stores import fish from overseas wholesale breeders in places like Thailand and the Orient and they raise them there most often in seawater mixed with minimal amounts of freshwater due to the scarcity/expense of fresh drinking water. Fish like this don't adapt easily to the home aquarium, so you'd want to finds a LFS where the owner either buys them locally bred or breeds them him/herself or find a breeder online who is happy to answer your questions. If you go with livebearers, get a trio (2 girls and a boy) of 1-2 different species (guppies and platies or mollies...although mollies usually prefer brackish water...as they are found in the wild in coastal areas like rivers and streams around Florida. Some types of mollies and
swordtails can get up to 5 inches, so personally I would keep those in a 20-29 gallon or bigger.) In a tank with lots of cover, livebearers will populate for you, meaning you buy fewer fish and their offspring will be born in your water conditions and will continue thrive for many generations in your well maintained aquarium.
The smaller species of tetras or danios could also do well for you (careful with the danios though, they are top dwellers mostly and in my experience they can jump out on you if you don't have a hood that finds well or a glass top with a hood/light on top.) When I worked at a big fish store 25 or so years ago, I always tried to steer the newbie aquarium hobbyists to choose 1-2 types of schooling fish (usually a bottom feeding fish and either a mid-tank or top tank feeder) and then maybe another easy fish like live bearers. One thing to know, barbs are quite fin nippy, so don't put them in with any longer finned fish or they will kill their neighbors (and
tiger barbs grow to about 4 inches, so the cute little guys in the store will eventually out grow a 15 gallon,
cherry barbs stay far smaller and are a little more peaceful, compared to tigers.) Cichlids that stay small could also be a choice, however they can be aggressive, particularly when they pair up and decide to set up housekeeping (often bullying and chasing off mild mannered tank mates that wander too close to their territory.) All in all, it depends somewhat on preferences...some people just want a peaceful and pretty tank with gentle fish and soothing swimming movement...others really enjoy a semI aggressive or even a very aggressive tank. Another tip,
angelfish like a tall tank over a long shallow one, due to their tall fin structure they are also happiest with water on the acidic side
pH of 7.0 or lower, they come from the Amazon basin and the water there is soft and a little acidic with lots of
tannins in the water (so, if you fall in love with those, you will soon have another tank...a pair of angels is very happy in a 20 high or 29 or larger tall tank, with a few small to medium
catfish depending on tank size, some neon or
cardinal tetras or other normal finned tetras...again, I'd avoid long finned varieties with angels because they can get nippy depending on their individual personalities.
I recommend getting 1 species at a time (I'd probably start with the bottom dwellers) and let them settle in for at least 2 weeks to a month before adding more fish. This gives the
beneficial bacteria time to adjust to the increasing bio-load. Sounds like a boring way to do it, but you are less likely to experience fish losses this way. Of course the store will try to sell you more...if they guilt trip you, just tell them that funds are tight and you will come back in a few weeks when you can afford more and head to the register. (That's another plus of buying online from a breeder, you can always just say thanks, but not thanks and go elsewhere if their is too much up-sale pressure.)
