Hi I have a 55g established planted tank. My readings are ammonia 0, nitrite 0, nitrate anything from 5 - 20, ph 7.6, have had these reading for weeks. The problem is the past 3 days I have lost 4 guppies, 2 harlequin rasboras, 2 neon tetras. I do 20% water change weekly and add prime and stability to my new water. My stock is
4 male guppies, 6 bloodfin tetras, 3 harlequin rasboras, 4 male swordtails, 4 zebra danios, 3 silver tetras, 1 algae sucker and 2 cory cats (sorry can't remember the type).
Any help would be great, the fish all appear healthy but before dying they hang around the back of the tank looking lethargic, oxygen shouldn't be a problem because I have an air curtain which is on 12 hours with the lights.
Not sure what other information you need but would appreciate any help; this situation is extremely distressing as I am not sure what has caused these sudden deaths