I have a 30 gallon tank fishless cycled and fully stocked for approximately two months.

My stock is:

6 cardinal tetras
6 harlquin rasboras
3 bolivian rams
6 sterba cory cats


water params:
ammo: 0
nitrite: 0
nitrate: 10-40 (changed every 3 days ~30% to maintain nitrates under 40)
temp: 76
pH: 8.0

Everything was happy for approximately 2 months. Last week I lost a tetra. Since then I have lost 3 more. The dead ones have no signs of disease. Nothing has changed with my water conditions so I'm not really sure what to do. Any help would be appreciated!