A few things here.
First, I would use a water conditioner with water changes just in case. You are using tap water which will have chlorine and possibly chloramine; while a water purifier may say it removes these, I would not want to chance it. And while I am not saying this is the issue, I will say that if there is chlorine in the water, you would expect to see fish at the surface as you describe. This is just one cause for this.
You may not need Prime regularly, depending what is in your tap water. The other issue is the filter/purifier. What exactly does this do? Is it a softener too?
The Flourish should not cause this, but I would add it in the early morning. Plants need nutrients during daylight when they photosynthesize. I always add any plant fertilizers just before th etank lights come on, or just after if I forget.
Again, not likely the issue, but...
It wouldn't hurt to leave the airstone in tonight; something seems to have happened over night, and we don't know exactly what, but this can't hurt for a night. The see what transpires tomorrow.
On the pH and fish, 7.8 is good for Endlers. The wood is not likely to change this, or very little. But what is your tap water GH and KH? Check the municipal water supply website for this, it is important.