Did my tank cycle or is the biofiliter dead?

History:

After 6 weeks of trying to get my tank to finish cycling I was at no
ammonia, but off the charts in nitrates and nitrites for nearly 3
weeks. Algea growth was starting to get sizable so I added a pleco
who quickly went to work.

Unfortunately, I think he brought an ich outbreak with him. Within 7
days 75% of my stock was dead. Devil-be-damned, I took an agreesive
approach to getting the water in order:

1) Ich Rid
2) Aquarium Salt
3) Melafix (nearly everyone was showing signs of fin rot)
4) Amquel

I added the Amquel to bring the nitrates and nitrites down and after
three applications over three days, those numbers finally dropped.

Now I'm a week without death and five days with no sign of ich. The
fish are happier looking and the water is clear to the eye.

___________________________
Situation:
Ammonia is spiking (~4.0 ppm) while feeding is very nominal. Nitrites
are non-existent. Nitrates about 15-20ppm. Temps are ~81; Ph ~7.0.
It seems like the tank is trying to start cycling again...is that
likely the case? If I've killed my bio agents, does this mean my
biowheel is dead and needs to be replaced?

Thanks!