Tetras chasing each other and playing or fighting/ mating?
Tetras are schoolinging fish, they need to be in a bare minimum school of 6 per species. (( percent of tetra need at least a 20 gallon tank. And it would only be large enough fo one species. Some species of tetra get too large to be in a 20 gallon tank. Some species of tetra do best in a larger then average size school. Schooling fish that are not in schools, or in tanks that are to small, will be stressed. Stressed fish are unpredictable, the most peacefull species may act aggressive or bite fins, or chase tankmates. Active or aggressive species may act lethargic or shy. FYI, your fish were never acting playfully!, they have lived an exhausted stressfilled year, and they may be too jaded at this point to change even if you did get a larger tank and more of the species. But that is what you need to do! Get a larger Tank and more of the same species, 6 is a bare minimum school, I dont know how many fish would be ideal, or what size tank would be minimal, because you never mentioned the species,
Tetras are schooling fish so i would think that this is aggressive behavior caused by a lack of tankmates. If you only have two i would suggest getting at least a couple more(of the same species of course). Look up a good school size for them and how many you can put in a ten gallon and figure out how many you should get.