Small PH fluctuation is not much of a concern without the change in water hardness at the same time. My tap water is also very soft. I do not see large swing in PH aside the time during fishless cycling with the buildup of high concentration of nitrate. I have used CO2 injection too.
By the way, I forgot to mention something from one of your earlier posts. You have said the filter media is quite small. To partially solve this issue, you can use more filter media or even just a piece of sponge in the place of carbon pad. In fact, a lot of us had removed the activated carbon pad, and in its place we use more of the filter media required for biological filtration. You can also use a small sponge filter at the intake of your filter to increase biological filtration. As a bonus, the sponge filter on the intake will block most of the debris. It can keep your filter clean, thus lower the maintenance requirement on the filter itself.
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20 gal tank - good mix of community fish ?
