I have run this diy citric acid /baking soda system before (bar type). You probably have a leak somewhere if u ran out in 2 days, (no pressure on the meter) or all of the acid flowed over to the soda side.
The ball on the acid side tube has a metal core. You'll have to get a strong magnet and use that to control how much acid can be drawn over. Usually you just need it to be about 1/4 inch below the surface.
As for which diffuser, the Riggs reactor would be the best option. A ladder type is the next best option. Ceramic diffusers /atomizers are not recommended as the diy cannot maintain enough pressure for long.
For a 60L tank, I would highly recommend u go pressurized. The diy requires u to constantly monitor it all the time, if the pressure drops, you'll need to readjust the needle valve to maintain your bps.
And with the fluctuating bps, you're asking for algae issues.
I know, I've been thru it
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