Question about planted aquarium and fishless cycling

So frustrated right now. I had another guppy dead last night and this morning a zebra danio. Last night I thought it must be the danios stressing the guppies out. The first 2 didn't seem to have any damage to their fins but the 3rd last night had most of her tail fin gone. But now a danio dead? It had a lot of red in the gil area and abdomen. There is one danio that is relentless about chasing the others.
Do you think it's the danios aggressive behavior causing the other fish to dye or my water? Findings today pH steady at 8.2, ammonia maybe 0.25 (I think I see a tent of green my kids say no it's just yellow), nitrites 0, nitrates 20-40 (maybe a little darker than the 20 but not as dark as the 40). Can an ammonia level of 0.25 be killing my fish? When I found the first two guppies dead it was maybe up to 0.5. I did a 30% water exchange at that time and it was enough to bring it back down to 0.