When the CO2 system shuts off does the Ph shift up a little ? Right now Ive got a Ph of 7 and a Kh of 3.Im still losing fish practically overnite.

If you're tralking about turning the CO2 completely off at night when the lights are off... depending on surface agitation, CO2 concentration and your KH, pH can "swing wildly" if you turn off the CO2 at night. I run my CO2 controller 24x7 - lights on or off - specifically to keep the pH stable.

If you're talking about the solenoid turning off the CO2 because the pH controller told it to, then it will depend on the "dead band" of your controller. I have mine set at .05 pH. That means if I have my controller set at, say 6.9 pH, the controller turns on the CO2 at 6.9 and shuts off at 6.85. If your dead band is set too wide, you can get a significant up and down fluctuation in pH as the controller turns the CO2 on and off.

Take care...

Tim

Your PH is not what is giving you a fish kill... it would have to be a super drastic PH swing to do that...it will stress them into disease but it usually will not kill overnight...

I do not understand what you mean by CO2 shutoff ? In any case (disconnecting/solenoid) you should never shut it down...the plants will take care of your PH if CO2 is controlled correctly.

I would look to ammonia spikes by way of damaged bacteria? A 15 gallon tank can be tricky with quick changes...Have you cleaned out your filter too well ? Or are you overfeeding this tank and the bacteria cannot keep up?
Test for Nitrites,Nitrates, Ammonia and get precise readings...
Ammonia is the only thing I know of that kills that fast.

Something in that water aint right ...

the lights.This is what I read to do! At night I thought the plants use O2 and repspire CO2 so they dont need any.Besides no photosynthesis is going on either.So yes, my solenoid is shutting off the gas for the nite.I dont have and never had ammonia or nitrate in the tank.I have a core of fish that seem fine no matter what other fish loss I had.Those are some rummy nose and pristella tetras and the ottos and cory cat.I lost all my rasbora but i think that was largely due when I had too much CO2 (Ph-6.0 and kh of 4 =over 100ppm CO2).Ive since adjusted it down to 10ppm .The plants are fine .Friday I replaced 2 more rummynose in which one died the next day and I added 5 more cardinal tetras which were eating that evening .2 died the next day and 3 died the following day.Yet the other 5 rummynose and 3 pristellas are fine.Maybe I should try another LFS.