The crushed coral is raising GH. Remove it. If you want to raise KH, use K2CO3. That said, there’s no reason to raise it unless you have certain fish that need a higher pH.
The pH of this tank was as low as 4.9
It will help all the plants. There’s some amazing journals recently with a few guys using low temperature to grow CO2 required plants without CO2. Amazing stuff really, drop to 74 and see what happens. I dropped my tank to 74 recently. Wish I could go lower and may just see what happens if I go to 72. Cautiously though!