Nitrite spike

I rescaped my 30L tank, as I wanted to try my hand at Blackwater. Stocked with a male betta, 9 (was 10) neon green rasbora, snails and shrimp. Everything went fine with the scape, reused the soil and refreshed it with new stuff. Added a sand layer and a "pathway" with gravel. Plants are all existing plants and elephant stone was in there before. The only things that are new is the wood, black sand and botanicals. I did boil the botanicals before hand as recommended. Yesterday I noticed the rasbora gasping at the top and I was missing one (found dead later on) so I immediately tested the water and the Nitrite is super high! Did a 50% water change, added an air stone as I originally thought it was an oxygen issue and added 5x dose of prime. I could tell immediately that the fish were happier so I recharged a Nitrite out bag and that has been in there since. Waking up this morning I notice the same behaviour from the rasboras, breathing from the surface. So tested again and the Nitrite is back up, so I repeated the water change and prime dosing. Fish seem to be much happier again but can someone help me work out why this is happening??? The only solution I can think of is that the nitrites are leaching into the water column from the existing soil.. so il be grabbing another bag of sand to increase the capping. Anyone got any other ideas?