My dad has a 55g set-up which has been running for around 2 yrs. His stock was primarely africans (not sure on all species, but he had a pair of electric yellow Labs. and a pair of red zebra)with a pleco. He recently lost everything except the pleco over a course of a couple weeks. He is routine in his water changes and I gave him my test kit to check ammonia (0), nitrite (0), nitrates (30)and Ph was a little low. What could have possibly caused such an outbreak of deaths?