1 teaspoon of KHPO4 should be about 6 grams, so 1/64 should be about 0.1 g.
Adding 0.1 g into 10g tank (38L) gives you concentration of 1.8 ppm PO4.
I don't say it's safe or not, but I don't know of any scientific paper which deals with the toxicity of phosphates on aquatic critters. That said, I know that even quite small amount of NO3 (1-5 mg/ℓ NO3) can cause increase in mortality with some sensitive kinds of invertebrates (in juvenile stages, fry, fingerlings or fish eggs). Adult fish and invertebrates can tolerate much higher NO3 concentrations (often more than 400 ppm). I would assume that similar results may apply for PO4.
Although 30 ppm NO3 and 3 ppm PO4 can cause no death of adults, it doesn't mean it is safe and harmless for eggs or fry. The increase of mortality of fish eggs or fry due to increased concentration of these nutrients is not something you can easily observe.