Is there a natural way to raise tank alkalinity

I have a 20g fresh water tank that is 3+ years old.
Our water is naturally soft and I have problems keeping the PH up. It is a
community tank and I would like to keep the PH around 7.0. It is currently
about 6.6.
I have done frequent water changes and I use a gravel filter to drain the
water.
The Nitrates are about 20 ppm.
I have a penguin bio filter which I change monthly and an Ehiem canister
filter left over from my 55g tank from years ago. The Ehiem filter I clean
about every 3 months. I cleaned it about 6 weeks ago.
Right now the tank is very clean.

3 weeks ago I added 4 fish and lost 3 of them plus 5 other fish.
I took my water into a fish store to be checked.
The PH is low which I know and so is the alkalinity.
A couple of years ago when I had PH problems before, I got advice to add
crushed coral and maybe some sea shells as a natural way to raise the PH.
This seemed to work for about a year. I recently added a few extra sea
shells to try to raise the PH.

Is there a natural way to raise the alkalinity? I am guessing that is part
of the reason why my PH is so low. The fish store folks are trying to sell
me some buffering chemicals which I want to resist if I can.

Thanks,
Mark