Best way to fix Low PH?

[Fellis]

I would love to get some advice.. 

I have a mildly low PH of 6.6. I would like to raise it to a more neutral level. I have a 20 gallon tank with tetras, livebearers, assassin snails, shrimp (unfortunately, they just died. I would like to reintroduce shrimp again) and live plants. I have read that crushed coral is a good solution to slowly and naturally raise the PH. I have a fluval 107 canister filter if that’s helpful. 

I have black/dark grey substrate and I would like to keep this aesthetic. My question is: 

1. what would you recommend to slowly and naturally raise the Ph while maintaining my aesthetic?  I know I can add the crushed coral to my filter, but I have read this can raise the PH quickly and it will run through the coral faster. What are my other options? 
2. what’s the best way to introduce the added substrate or natural items to my tank? 

Edited November 22, 2020 by Fellis
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[Daniel]

Crushed coral is probably your best bet.

I be surprised if putting crushed coral in your canister filter would cause it to rise too quickly for your system to handle.

My pH moves quite a bit over the course of one day when there is a lot of photosynthesis happening.

And below are further daily pH movements as carbon dioxide is consumed during the day causing the pH to rise and then released at night, causing the pH to fall as photosynthesis run in reverse in the dark.

[Fellis]

Thanks Daniel! This is helpful.