10 gallon tank using Aqueon carbon filter system. I did a fishless cycle for about 2 months using ammonia powder designed for cycling and API test strips. After dosing to 4 ppm ammonia and testing results were zero for ammonia and nitrites within 24 hours, I considered my tank cycled. Nitrates also tested to make sure present at appropriate levels. I did this multiple times to make sure before buying fish. My mistake was overstocking, I think. We got 6 guppies and 3 Cory cats (all babies). After getting fish, I tested and ammonia at zero but nitrites were sky high (deep purple). Made sure not to over feed. Started doing huge water changes. Testing after water changes still showed no improvement with nitrite levels. After a week all the fish are dead. How do I recover so that all my efforts in cycling were not for nothing. With no fish now, do I go back to dosing with ammonia? And once levels are right do I just get a couple fish to avoid this again? Thank you!