Lemon tetras dieing very fast

[ryouexp1]

I originally bought a school of 18 lemon tetras about 2 years ago. I added 10 red tetras a few months later. The lemons bread three time and the school rose to about 25. I have a very large planted tank. Ph 6.5 temp 78 degrees, all other parameters are normal. The tank is very stable. Last week I lost my pleco which has been around for about 8 years, then I lost a red tetra, then a couple of more died then I lost almost all my lemons in the last two days. I have 3 reds and 7 lemons left. No other fish seem to be affected..
I have never had this happen before does anyone have any idea what could be happening?
Oh, some of the dead ones seem to have some something on their sides but others look perfect before they die. Some bodies are intact and some rot withing hours...

 

[Rbishop]

What size tank?
Current pH, ammonia, nitrite, nitrate readings from a good liquid test kit?
Current stocking, exactly by numbers and type?
What is your water change/maintenance routine?

 

[Slappy*McFish]

Some years ago, I experienced the sudden dying of a group of Lemons, as well. I attributed it to bullying from a group of Serpaes and a RTBS.

 

[ryouexp1]

Ph 6.5 and stable, no ammonia,or nitrite, nitrate at 10-20ppm, phos 1-2ppm, KH 5-6, Gh 5-6.
4 sinamonkie (sp?) loaches, 3 corey cats, one large busy nose pleco, on female checkerboard, one loan flame tetra, two siamonsis (sp?) algae eaters and 5 odos. I have a low fish stock since it's a primarily a plant tank. It's a very stable tank and I do water changes every one to two weeks. At this point all the lemons are dead and I have three red tetras left. All the other fish are fine, even the odos.