I have a 100 liter tank that I started with 5 Black Tetras and then added 4 Serpae's to. The aquarium now consists of the original 9 fish plus 4 Albino Corys, 1 Rainbow Shark, 2 Powder Blue Gouramis, 1 Loach, 5 Penguin Tetras an Apple Snail and several live plants. All the fish do well together. The only conflicts are within the types. The Serpaes chase each other around, (as do the Black Tetras and Penguins), but I have not witnessed any aggression towards the other occupants. The chasing never seems to result in actual contact, and there is no evidence of fin-nipping either, which I am pleased by. I have done almost nothing to the water for treatment. I keep the pH around 6.7, temperature at 29°C and do 25% water changes every 4 or 5 weeks. I feed a combination of flakes, freeze-dried shrimp, and algae wafers (for the loach and snails).
I've read a lot of comments saying Serpaes are "fin-nippers". Actually, when they're getting enough to eat, mine don't bother anyone. It's when I've missed a couple feedings that they harass other fish. My suggestion is to make sure your Serpaes get enough food. Breeding Serpae Tetras is actually quite easy. Put a male and a female in a 40 liter tank with a couple plants and a rock or two. Filter the water through peat and raise the temperature to about 28°C. By morning they'll have laid eggs either on the rock, the gravel, the plants, or the heater (or just scattered them all over the tank). As with most Tetras, they completely ignore the eggs unless they want to eat them. Females are a little rounder and their swim bladder goes down sharply on the side of their bodies. Males have a more gradual swim bladder curve and will sometimes have mock-battles with other males.
I wanted to note that many people who have these fish and experienced problems with them usually don't have them in big enough schools. You can't have three or four of these fish and expect them to be okay. They like to be in groups of 8 or more. Many people are keeping WAY too few tetras at a time. In general, at least six fish need to be purchased at a time if the fish is a schooling fish.
I noticed there were a lot comments on these fish being aggressive or tale nippers. Personally I think there behavior is very peaceful. We have them in three tanks. The first has five serpae tetras, two female Bettas, and a blue platy. The serpaes leave the other fish alone and just school. The other is a 175 liter bow front with 5 serpaes, 4 lemon tetras, long finned danios and two pearl gouramis. The third is a 110 liter with 5 serpaes, 3 lemons, 4 bloodfins, 7 swords, some babies, 2 platies, 3 longfinned danios. In all of these tanks they swim peacefully, do not nip tails and are a beautiful addition.
I have a tank with a pair of gouramis, 6 harlequins, 7 neons, 3 blue rams, 2 suckers and 2 loaches. Everything was great until I introduced 4 serpae tetras. They proceeded to nip the skin off the sides of 2 harlequins and one neon tetra, thus killing them. I have seen them nip at the hanging feelers of the gourami and chase a blue ram around the tank. I have now removed the serpaes. Mine were definitely not docile and good community fish.
These fish need to be kept in groups. I made the mistake when I was a beginner, putting a lone Serpae into my community tank, and it bullied my Bronze Corydoras so much, the catfish eventually had nipped fins, and developed white spot due to being so stressed. These tetras will do better in more active communities, away from slow moving fish.