When seining fishes in Paraguay, I never found many serpa tetras in the same place. They seem to school, but usually with other species. A normal catch, from say 3 x 5 m, could be 1000 Serrapinnus spp. (2-5 different species of silvery tetras with a black caudal peduncle spot), 300 Aphyocharax anisitsi (bloodfin tetra), 50 Aphyocharax paraguayensis, 50 Astyanax, 20 Gymnocorymbus ternetzi (Black tetra), 30 Pyrrhulina australis, and only about 5 Hyphessobrycon callistus (plus a few different cichlids and catfishes). Unfortunately, I never observed the behaviour among the tetras - if the different species kept together - but there was no such indication. Only occasionally there were a significant number of serpa tetras (maybe 50) in the same place. This doesn´t explain the aggressive behaviour to other species, but well within the species.
I've had 3 of these tetras in the long finned variety. My experience is the long fin variety are not as brutal as the normal ones. Might be a good alternative as they are just as beautiful. Don't be surprised to see their fins in shreds as they dispute ranks amongst themselves.
I have added a group of five Serpae Tetras to a community tank which had a small group of Flame Tail Tetras. After joining the tank, they got along as if one species (similar size) and began jockeying for position in a group. With mixed aggression - not attack - the flame tails joined in for position in the same group. First dinner was mildly competitive - including positioning for post-digestion. Now they act as one group. Reddening was almost instantly as they began asserting their ranks. Tank shared with two Oto's, 3 Albino Corys. Would recommend them for a group show anytime.
I had gotten 3 serpaes for my tank and they nipped the heck out of my bloodfins, cardinals, and my female bettas. I then took out my female bettas and put in my male betta. Once again they ended up nipping at his fins. They would wait until he went up for air or until I turned their light off to attack him. After a few days I had to get rid of them. Ended up giving them to my boyfriend, whose fish were a bit more aggressive. Needless to say, I'm not a big fan of serpaes.
I keep 4 Long-Fin Serpae Tetras, 2 Bleeding Heart Tetras, 1 large Clown Loach, and 1 Otocinclus. The Oto and the Tetras have been happy and healthy for almost 2 years now, and the Loach for about a year. It is a 100 liter tank with natural stones. I feed them flake food twice a day and drop some algae pellets for the Oto and Loach once or twice a week. We clean the tank (with vaccuum) once or twice a month and change the arrangement of plants and stones about twice a year. They've never become ill and aren't aggressive at all. Of course, they are the smallest fish in the tank but the most numerous. The tetras are the nicest fish I've ever had.
I've had Serpae's in my community tank for about 5 years (not consecutively), and although they do tend to jockey for position amongst themselves, I've never had a problem with them becoming aggressive towards their tankmates. I've kept them with danios, rasboras, neons, gouramis and cories without any issues. I think the important thing is to buy them when they're young in a group of at least 6, and put them with larger fish off the bat. In my experience that seems to keep them relatively peaceful towards other species, even when they get larger and you add smaller fish down the road. They're truly beautiful fish when they're happy - mine seem to like a heavily planted tank; any time I've added a red-leaved plant their colors have brightened and I actually got to witness them spawn, which was really amazing. When I add tubifex worms for my gouramis or algae pellets for the cories, they hang around waiting for the little bits of food to come flying off and enjoy the banquet.
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