I've got a bog standard 2 feet tank with a cannister filter and an air pump. Set up is almost three months old. Water parameters I haven't got a clue, those test kits are expensive!!!

My community tank has:
6 1-inch harlequin danios
4 1.5-inch serpae tetras
6 1.5 inch rummy nose tetras
3 1.5 inch cardinal tetras
3 1.5 inch pepper cories
3 1.5 inch albino cories

Recently added 10 0.5 inch neon tetras (couldn't resist )
No problems for the first two days, but noticed this morning b4 work that two of them little ones are hovering near the surface with their tails completely ripped off!!! No fin damage, no white spots etc! Just no tail!!!

Who's the culprit (or is it me cause I've overstocked???) I suspect the serpae's but b4 this they only nip their own kind... quite badly at that...

Any help? Thanks.