I wouldn't say that at all. It may be true with freshwater, but I don't do fw so I don't have a clue there.
Stable alkalinity is good, but it can be too high or too low. Just because your alkalinity is stable doesn't mean it's good. If you have a alkalinity between 8-12 dKh, then you should have stable pH... regardless of what the number is. While not accurate, you can think of alkalinity as a pH 'reserve'... more alkalinity will allow your pH to resist its normal tendency to drop.
But if your pH is below 7.8 or so (even if you alkalinity is stable), you might want to figure out why that is. If you're not seeing any ill effects, then it's probably better to just leave it alone. But there's an underlying reason why it's low, and if you CAN fix it, you probably should.
I don't want to de-rail the thread, but since it's asking about pH I suppose it is on topic and useful to the OP as well.
I do have a FW tank. I have a kh and gH test kit. The way they work is you keep adding drops until you see a color change, and then multiply the number of drops by 20 or 10, I forget which is for which.
Either way, I usually see the color on the first or second drop of kH, which means it is VERY low, and I usually have a gH (which is also alkalinity right?) of around 280-300 which is VERY Hard.
From what I understand, if your kH is high it means you have a high buffering capacity (whatever that means) and you won't see pH shifts, even from CO2 injection (which is supposed to bring your CO2 down by 1 point). However, at the same time, I've also learned that shifts of pH from CO2 don't affect your fish, and that you don't need to worry about shifts in pH, but rather it is swings in kH that can be harmful.
Needless to say I'm very confused here.
I've asked if I should add baking soda to raise my kH but most said no, and that a very low kH is fine, it's just swings in kH that will harm the fish. Well what causes kH to swing? If it's adding baking soda then I definitely want to avoid it. So I'm really asking here, for someone like myself, (FW, planted tank, w/ CO2 injection) is an almost non-existent kH okay, or should I add baking soda to get it up higher?