Cardinal'ss are sensative fish at first, once they acclimate they are very hardy, I keep them in my 29g with Florabase, KH of 1 to 2, ph 6.0.
You have a KH2 out of the tap? what is happening is once the C02 kick's on it is driving the 02 out, while also lowering the ph rapidly, low ph is fine, they like it, they need the 02.
Raising the spray bar was a very wise thing, no need for airstone, I use lilly pipes, so every evening when lights off I raise the pipe to surface, in the morning, I lower it again, while maintaining good C02.
Cardinal's also love soft acidic water! low ph high 5's to low 6's.
Once you get a better feel for the cardinals with a low KH you won't need the baking soda.
How are you diffusing C02? I would suggest a glass diffusor, you can get much better diffusion with less C02 than reactor's
So, in a nutshell, use the spraybar or pipe to strike a balance anytime it is needed, with surface aggitation, at night, raise the bar till it gives a light splash on surface.
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