Praecox rainbows surface gulping!

[Snowfire]

Tank size 90g
Ammonia-0
Nitrite-0
nitrate<10
pH-7

Stock:
6 now 5 praecox rainbows,
5 Angelfish(mostly small)
8 platys
2 SAE

I bought 6 praecox rainbows about 3 weeks ago, they've been doing fine until last night. As I was going to bed last night, one of them was gasping at the surface, everyone else looked fine so I didn't give it too much thought. This morning one was dead!
I did a 50% pwc this morning and the rainbows have all been acting strange since, tightly schooled but not swimming around, didn't eat
Just going to bed now and another is at the surface gasping.
I just don't know what's going on... Any ideas?

 

[blert]

How's your aeration? Are you injecting CO2?

 

[Snowfire]

I don't use co2, I don't have much aeration thinking about it, just surface disturbance from the spray bar. Could that cause them to die?

 

[blert]

Not sure. As long as you have good movement on the surface it should suffice but it couldn't hurt to add an air stone if you have an air pump lying around. If not, try pointing your spray bar up a little bit more to get the water moving on the surface.

 

[Snowfire]

Ok, I've just twisted the spray bar to move the water surface as much as possible. I do have a air pump somewhere so I'll look to buy a air stone tomorrow and get it running.
Thanks for your help!