When I speed cycled my 10G, I didn't CHANGE the water. I only started with about 5-6G and kept adding dirty water from the betta tanks.
I pumped up the heat to the 80's but the tank was ACTUALLY 98 degrees for 4-5 days.
Bud, do you have anything that will absorb the nitrates?
I'm not crazy about the filter you're using. I typically use the HOB style filters. In tank ones are troublesome to maintain.