Tetras dying fast -- what to replace them with?

[m0rl0ck]

Its a 29 gal heavily planted tank. The six diamond tetras became 7 when unbeknonst to me they spawned in the tank and only one of the fry lived. Lately a tetra every two to three weeks has been swimming erraticaly, turning white and dying in a matter of a day or so. Now I'm down to 4. I treated these fish a while ago for what I thought was a parasite infection and they all puled through, but whatever this latest malady is it kills fast and I'm swearing off tetras for good, tired of watching them die.

The tank also contains 6 turquiose rainbow fish and 5 otos and a few ghost shrimp.

What are some good tank mates for the rainbow fish?

 

[Lauren]

what are your perameters? I'd figure out what is killing off your tetras before replacing them.

 

[m0rl0ck]

My guess is NTD is killing them. Theres also a flagfish in the tank that I forgot to mention.
Everything thats been in the tank and is currently in the tank are doing fine. I had some neons about 10 years ago that displayed the same behavior / symptoms and went through dosing of everything I could think of.
I seem to have no luck with tetras. If you can give me a reasonable suggestion as to diagnosis / treatment for this bunch I'll take a shot at saving them. Met didnt work.

EDIT -- Sorry I didnt answer your question. I have no idea what my water parameters are. I change about 30% of the water a week and the tank has 250 gph filtration and nothings died and rotted lately. I do have diy CO2 which may be lowering the ph and I add 1/4 potassium chloride and 2tsp of liquid plant fert every other change.
Tap water around here is around 7.4 PH. Thats all I really know about my water parameters.