well if you are looking for a little difference, you could try to breed Bettas as Who-Izzet suggests. Get a male and female and you should house them in different containers until they are ready to breed. Feed them lots of good food and put them together provided that there is room for the female to hide. The male builds a bubble nest near the surface so either turn down your filter intake or provide some floating plants that will keep the bubble nest together. (your 5 gallon could serve that purpose). Be warned, bettas lay a lot of eggs!!! since you don't have a bigger tank, they will all died unless you remove most of the fry. its not very nice but u def cant support 200+ fry in a 45 liter. Its hard to keep the babies alive too...but u can experience their unique bubble nest breeding behavior. If you want the fry to survive, i would just save a few. They would have a better chance of growing up. But if that sound cruel to you, then i wouldn't suggest breeding anything else besides guppies and small egg spraying fish in your limited tank space. And no, don't try convicts because they need at least a 20 gallon for a breeding pair.