disappearing

I have a Juwel Rio 125 litre (T5 lighting, I think; def not LED), “pimped” internal Jewel filter with 2kg of biogravel. Sand substrate over Tropica aquarium soil. . pH 7 - 7.5. Tap water source is hard (London) - DH14. Amonia, nitrate and nitrite levels all fine. Pond snails. No fish yet. The...