8ppm is really bad. Sorry.
Can you post a pic of your tank, the filter part of your tank and your test results? How many fish are in the tank now?
Keep doing the water changes. Skip a day feeding them.
Can u buy an air pump? If yes, Mr. Fisher sells seeded sponge filters in the classifieds. The sponge filter will need tubing and an air pump, but if you use a seeded filter your aquarium should stabilize very rapidly.
I'm going to go google some things about the fluval 6. I'll edit if I get any.
Edit: I found a lot of posts about nitrogen cycle difficulties with the fluval edge 6 aquarium. That doesn't mean there is a problem with the tank, but it does mean that a picture of your filter system on that tank can get you better help. There might be more room for more bioMax. One of the more experienced AA members can look at the pic and advise you.
From what I've been reading I think adding an air pump and (seeded) sponge long term would be of great assistance for stability.
Edited: The problem you describe. Just unplug your filter and definitely always unplug your heater when doing your water change.