So I have been cycling a 20gallon tank for just over two weeks. First week I added Seachem Prim and then Stability every day for 10 days. Added ammonia, shook it in glass jar, no bubbles, said no additives, assume it is good. Ammonia readings were between 5-8ppm at the start. Now they are reading about .5ppm, nitrites and nitrates show no readings at all yet. I thought I would try and help the cycle along by adding 2 Zebra Danios, a dose of prime and dose of stability. Both Danios were very active at the store and in the bags on the way home (5 minute drive). While floating them in the tank for 15 minutes, 1 sank to bottom of bag and looked to have died. Pet store called and said they are known to faint so put in tank and monitor, other one swimming around happily. "fainited" fish is still on bottom of tank on its side, pretty sure its dead, other won is now hanging out at the surface of tank hiding behind hob filter. Penguin 150 filter, heater set at 78f, gravel in bottom of tank and fake plants. Suggestions?