You can try mixing sands designed to buffer, like Aragamax.
I'd ignore the advice about not using crushed coral because there's none in the lake. There are no HOB filters either.
Test your level of 'buffers' with a KH test kit. You shouldn't consider pH alone, as it goes hand in hand with KH, sometimes called alkalinity or buffers. Your pH may be ok at the moment, but natural processes consume and lower KH, and if it gets too low, pH can crash. I'd not make any decision about needing buffers based on pH test results alone, you can't. If you don't want to add chemicals, then try the crushed coral or sands that buffer while testing KH regularly to see if it has the needed effect. I tested Aragamax sand with distilled water and was surprised by the dramatic effect it had. I used to think that it didin't make much difference.
See this article in the forum library for more info =>
Practical Water Chemistry
Xeno's, paracyps, calvus or comps. Keep one of the brichardi, singles do ok. You can keep the single juli also. Brevis are another possibility.