Hi, I set up a 10l nanotank at Christmas and it finished cycling on Thursday 23rd Jan. It has soil substrate, dragon stone, java moss, java fern and dwarf grass. On Friday 24th I completed am 80% water change and bought 6 phoenix rasborah fish. All was well on Friday and Saturday. I was missing a fish Sunday night as in I couldn't find it and on Monday morning 2 fish died without any strange behaviour. I tested the water and had zero ammonia and nitrite but approx 40 to 60 ppm nitrate. I immediately did a 70% water change dechlorinating the water and 1 of the 4 fish looked sluggish. Sure enough this morning it was dead. So that's 3 fish dead in 4 days. I feed them flake once a day that I ground into a powder. Any advice on what could have happened?