I have a 90 litre tank, currently stocking 1 angel fish, 1 pearl gourami, 4 silvertip tetras, 5 corydoras, 1 siamese algae eater and 2 bristlenose plecos. About 3 weeks ago i purchased 6 cardinal tetras. They settled in fantastically and thrived for the first 2 weeks. Then all if a sudden i wake...