can i buffer my water with baking soda?

Yes,but seachems marine buffer IMO is nuch better, and what's the PH? may not be nessesary to fix it if it's a fish only tank.

what's wrong with your water just the way it is. in my opinion, your pH is fine, as is your hardness. playing chemist with your water is not as simple as adding this or that chemical. there are numerous inter-related parameters and effecting a change in one almost always has an impact on the others, often to the detriment of the fish. moreover, your water has an inherant chemistry which it will strive to return to once the effect of your chemical addition has worn off. this 'yoyo' effect is far more stressful to fish than having a chemistry which is slightly or even moderately different from that which a book may suggest as necessary.

moreover, your water has an inherant chemistry which it will strive to return to once the effect of your chemical addition has worn off.

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Yes and no. Biological processes both consume carbonate and also neutralize it, depleting the buffering capacity of the water. If I am translating the units correctly (~100gh/50), we are looking at an alkalinity of ~2 mEq/L, which is very low. This means the buffering capacity of the water is reduced, and it will not be able to resist swings in pH very well. The bicarbonate in baking soda is an excellent supplement. Start small, a tablespoon or two per day, and see what happens to your alkalinity/hardness.

The bigger question is what is neutralizing your buffers? How often are you doing water changes? Is there a lot of decomposing material in the tank? How many fish, and what are you feeding?

PH is on the low side 8.4 is right, and with low PH levels, most of the bad algae growth increases.