Hello,
This is the establishement of bacteria working on nutrients, likely laden in rock and sand. This shall pass, I would utilize inverts in this case to help, turbo snails work best, astrealas and other variety to also help. I would most certainly bring your values of NO3 up, this will help overall, being zero is not advisable long term. Phos is good for now, perhaps adding a touch more phosphate minus will help. Careful with Zeomix, it can bottom out nitrates with use of NP Pro and Pro Bio S, so you could do one of two things to help lift nitrates. Feed fish more, and/or lower use of NP Pro and Pro Bio S. Cheers